gnu: fp16: Fix build on i686-linux.

* gnu/packages/patches/fp16-implicit-double.patch: New file.
* gnu/local.mk (dist_patch_DATA): Adjust accordingly.
* gnu/packages/maths.scm (fp16)[source](patches): Add it.

gnu/local.mk

@@ -1112,6 +1112,7 @@ dist_patch_DATA =						\
   %D%/packages/patches/foobillard++-pkg-config.patch		\
   %D%/packages/patches/foomatic-filters-CVE-2015-8327.patch	\
   %D%/packages/patches/foomatic-filters-CVE-2015-8560.patch	\
+  %D%/packages/patches/fp16-implicit-double.patch		\
   %D%/packages/patches/fp16-system-libraries.patch		\
   %D%/packages/patches/fpc-reproducibility.patch		\
   %D%/packages/patches/freedink-engine-fix-sdl-hints.patch	\

gnu/packages/maths.scm

@@ -7690,7 +7690,8 @@ when an application performs repeated divisions by the same divisor.")
                 (sha256
                  (base32
                   "05mm4vrxsac35hjf5djif9r6rdxj9ippg97ia3p6q6b8lrp7srwv"))
-                (patches (search-patches "fp16-system-libraries.patch"))))
+                (patches (search-patches "fp16-implicit-double.patch"
+                                         "fp16-system-libraries.patch"))))
       (build-system cmake-build-system)
       (arguments
        `(#:imported-modules ((guix build python-build-system)

gnu/packages/patches/fp16-implicit-double.patch

@@ -0,0 +1,23 @@
+Prevent implicit conversion of float to double to avoid precision
+error on i686.
+
+  https://github.com/Maratyszcza/FP16/issues/20
+
+Taken from Debian:
+
+  https://salsa.debian.org/deeplearning-team/fp16/-/blob/master/debian/patches/ftbfs-i386.patch
+
+Index: fp16/include/fp16/fp16.h
+===================================================================
+--- fp16.orig/include/fp16/fp16.h
++++ fp16/include/fp16/fp16.h
+@@ -228,7 +228,8 @@ static inline uint16_t fp16_ieee_from_fp
+ 	const float scale_to_inf = fp32_from_bits(UINT32_C(0x77800000));
+ 	const float scale_to_zero = fp32_from_bits(UINT32_C(0x08800000));
+ #endif
+-	float base = (fabsf(f) * scale_to_inf) * scale_to_zero;
++	const volatile float base_inf = fabsf(f) * scale_to_inf;
++	float base = base_inf * scale_to_zero;
+ 
+ 	const uint32_t w = fp32_to_bits(f);
+ 	const uint32_t shl1_w = w + w;