gdb: Enable finish command and inferior calls for _Float16 on amd64 and i386.

gdb: Enable finish command and inferior calls for _Float16 on amd64 and i386.

Values of type _Float16 and _Float16 _Complex can now be used on CPUs with
AVX512-FP16 support. Return values of those types are located in XMM0.
Compiler support for gcc and clang is in progress, see e.g.:
https://gcc.gnu.org/pipermail/gcc-patches/2021-July/574117.html

gdb/ChangeLog:
2021-07-21  Felix Willgerodt  <Felix.Willgerodt@intel.com>

	* amd64-tdep.c (amd64_classify): Classify _Float16 and
	_Float16 _Complex as AMD64_SSE.
	* i386-tdep.c (i386_extract_return_value): Read _Float16 and
	_Float16 _Complex from xmm0.

gdb/testsuite/ChangeLog:
2021-07-21  Felix Willgerodt  <Felix.Willgerodt@intel.com>

	* gdb.arch/x86-avx512fp16-abi.c: New file.
	* gdb.arch/x86-avx512fp16-abi.exp: New file.