gnu: gcc-5.1, gcc-toolchain-5.1, libstdc++-doc-5.1: Rename to *-5.

This reflects the new version numbering scheme for GCC 5 and up.
See <https://gcc.gnu.org/develop.html>.

* gnu/packages/gcc.scm (gcc-5.1): Rename to ...
  (gcc-5): ... this.
  (libstdc++-doc-5.1): Rename to ...
  (libstdc++-doc-5): ... this, and use gcc-5.
* gnu/packages/commencement.scm (gcc-toolchain-5.1): Rename to ...
  (gcc-toolchain-5): ... this, and use gcc-5.
* build-aux/hydra/gnu-system.scm (%core-packages): Use gcc-5.

build-aux/hydra/gnu-system.scm

@@ -101,7 +101,7 @@ SYSTEM."
   ;; Note: Don't put the '-final' package variants because (1) that's
   ;; implicit, and (2) they cannot be cross-built (due to the explicit input
   ;; chain.)
-  (list gcc-4.8 gcc-4.9 gcc-5.1 glibc binutils
+  (list gcc-4.8 gcc-4.9 gcc-5 glibc binutils
         gmp mpfr mpc coreutils findutils diffutils patch sed grep
         gawk gnu-gettext hello guile-2.0 zlib gzip xz
         %bootstrap-binaries-tarball

gnu/packages/commencement.scm

@@ -830,7 +830,7 @@ and binaries, plus debugging symbols in the 'debug' output), and Binutils.")
 (define-public gcc-toolchain-4.9
   (gcc-toolchain gcc-4.9))
 
-(define-public gcc-toolchain-5.1
+(define-public gcc-toolchain-5
-  (gcc-toolchain gcc-5.1))
+  (gcc-toolchain gcc-5))
 
 ;;; commencement.scm ends here

gnu/packages/gcc.scm

@@ -331,7 +331,7 @@ Go.  It also includes runtime support libraries for these languages.")
                 "0zmnm00d2a1hsd41g34bhvxzvxisa2l584q3p447bd91lfjv4ci3"))
               (patches (list (search-patch "gcc-libvtv-runpath.patch")))))))
 
-(define-public gcc-5.1
+(define-public gcc-5
   (package (inherit gcc-4.9)
     (version "5.1.0")
     (source (origin
@@ -604,8 +604,8 @@ using compilers other than GCC."
 (define-public libstdc++-doc-4.9
   (make-libstdc++-doc gcc-4.9))
 
-(define-public libstdc++-doc-5.1
+(define-public libstdc++-doc-5
-  (make-libstdc++-doc gcc-5.1))
+  (make-libstdc++-doc gcc-5))
 
 (define-public isl
   (package