Revert "gnu: Default to GCC 5."

This reverts commit e3d0fcbf7e.
This is a temporary measure until GCC 5 builds on ARM, see:

  https://gcc.gnu.org/bugzilla/show_bug.cgi?id=71399

gnu/local.mk

@@ -492,7 +492,6 @@ dist_patch_DATA =						\
   gnu/packages/patches/gawk-shell.patch				\
   gnu/packages/patches/gcc-arm-link-spec-fix.patch		\
   gnu/packages/patches/gcc-cross-environment-variables.patch	\
-  gnu/packages/patches/gcc-libiberty-printf-decl.patch		\
   gnu/packages/patches/gcc-libvtv-runpath.patch			\
   gnu/packages/patches/gcc-5.0-libvtv-runpath.patch		\
   gnu/packages/patches/gd-CVE-2016-3074.patch			\

gnu/packages/commencement.scm

@@ -170,26 +170,6 @@
                     ,cf)))))
      (inputs %boot0-inputs))))
 
-(define libstdc++-boot0
-  ;; GCC's libcc1 is always built as a shared library (the top-level
-  ;; 'Makefile.def' forcefully adds --enable-shared) and thus needs to refer
-  ;; to libstdc++.so.  We cannot build libstdc++-5.3 because it relies on
-  ;; C++14 features missing in our bootstrap compiler.
-  (let ((lib (package-with-bootstrap-guile (make-libstdc++ gcc-4.9))))
-    (package
-      (inherit lib)
-      (name "libstdc++-boot0")
-      (arguments
-       `(#:guile ,%bootstrap-guile
-         #:implicit-inputs? #f
-
-         ;; XXX: libstdc++.so NEEDs ld.so for some reason.
-         #:validate-runpath? #f
-
-         ,@(package-arguments lib)))
-      (inputs %boot0-inputs)
-      (native-inputs '()))))
-
 (define gcc-boot0
   (package-with-bootstrap-guile
    (package (inherit gcc)
@@ -275,13 +255,14 @@
                ("mpc-source" ,(package-source mpc))
                ("binutils-cross" ,binutils-boot0)
 
-               ;; The libstdc++ that libcc1 links against.
-               ("libstdc++" ,libstdc++-boot0)
-
                ;; Call it differently so that the builder can check whether
                ;; the "libc" input is #f.
                ("libc-native" ,@(assoc-ref %boot0-inputs "libc"))
-               ,@(alist-delete "libc" %boot0-inputs))))))
+               ,@(alist-delete "libc" %boot0-inputs)))
+
+     ;; No need for Texinfo at this stage.
+     (native-inputs (alist-delete "texinfo"
+                                  (package-native-inputs gcc))))))
 
 (define perl-boot0
   (let ((perl (package
@@ -901,10 +882,10 @@ and binaries, plus debugging symbols in the 'debug' output), and Binutils.")
   (gcc-toolchain gcc-4.8))
 
 (define-public gcc-toolchain-4.9
-  (gcc-toolchain gcc-4.9))
+  (gcc-toolchain gcc-final))
 
 (define-public gcc-toolchain-5
-  (gcc-toolchain gcc-final))
+  (gcc-toolchain gcc-5))
 
 (define-public gcc-toolchain-6
   (gcc-toolchain gcc-6))

gnu/packages/gcc.scm

@@ -352,8 +352,7 @@ Go.  It also includes runtime support libraries for these languages.")
               (sha256
                (base32
                 "1ny4smkp5bzs3cp8ss7pl6lk8yss0d9m4av1mvdp72r1x695akxq"))
-              (patches (search-patches "gcc-5.0-libvtv-runpath.patch"
+              (patches (search-patches "gcc-5.0-libvtv-runpath.patch"))))
-                                       "gcc-libiberty-printf-decl.patch"))))
     ;; GCC 5 ships with .info files, so no need for Texinfo.
     (native-inputs '())))
 
@@ -372,7 +371,7 @@ Go.  It also includes runtime support libraries for these languages.")
 
 ;; Note: When changing the default gcc version, update
 ;;       the gcc-toolchain-* definitions accordingly.
-(define-public gcc gcc-5)
+(define-public gcc gcc-4.9)
 
 (define-public (make-libstdc++ gcc)
   "Return a libstdc++ package based on GCC.  The primary use case is when

gnu/packages/patches/gcc-libiberty-printf-decl.patch

@@ -1,28 +0,0 @@
-This patch makes the exeception specifier of libiberty's 'asprintf'
-and 'vasprintf' declarations match those of glibc to work around the
-problem described at <https://gcc.gnu.org/ml/gcc-help/2016-04/msg00039.html>.
-
-The problem in part stems from the fact that libiberty is configured
-without _GNU_SOURCE (thus, it sets HAVE_DECL_ASPRINTF to 0), whereas libcc1
-is configured and built with _GNU_SOURCE, hence the conflicting declarations.
-
---- gcc-5.3.0/include/libiberty.h	2016-04-23 22:45:46.262709079 +0200
-+++ gcc-5.3.0/include/libiberty.h	2016-04-23 22:45:37.110635439 +0200
-@@ -625,7 +625,7 @@ extern int pwait (int, int *, int);
- /* Like sprintf but provides a pointer to malloc'd storage, which must
-    be freed by the caller.  */
- 
--extern int asprintf (char **, const char *, ...) ATTRIBUTE_PRINTF_2;
-+extern int asprintf (char **, const char *, ...) __THROWNL ATTRIBUTE_PRINTF_2;
- #endif
- 
- /* Like asprintf but allocates memory without fail. This works like
-@@ -637,7 +637,7 @@ extern char *xasprintf (const char *, ..
- /* Like vsprintf but provides a pointer to malloc'd storage, which
-    must be freed by the caller.  */
- 
--extern int vasprintf (char **, const char *, va_list) ATTRIBUTE_PRINTF(2,0);
-+extern int vasprintf (char **, const char *, va_list) __THROWNL ATTRIBUTE_PRINTF(2,0);
- #endif
- 
- /* Like vasprintf but allocates memory without fail. This works like