core: aarch32: Fix standard for _Float16 in CONFIG_FP16 help message

Both Clang[1] and GCC[2] says that _Float16 type is defined by
ISO/IEC TS 18661-3:2015 not the IEEE 754-2008. Fix help message under
FP16 config option.

[1] https://clang.llvm.org/docs/LanguageExtensions.html#half-precision-floating-point
[2] https://gcc.gnu.org/onlinedocs/gcc/Half-Precision.html



Signed-off-by: Patryk Duda <pdk@semihalf.com>