Support -fexcess-precision=16 which will enable FLT_EVAL_METHOD_PROMOTE_TO_FLOAT16...

Support -fexcess-precision=16 which will enable FLT_EVAL_METHOD_PROMOTE_TO_FLOAT16 when backend supports _Float16.

gcc/ada/ChangeLog:

	* gcc-interface/misc.c (gnat_post_options): Issue an error for
	-fexcess-precision=16.

gcc/c-family/ChangeLog:

	* c-common.c (excess_precision_mode_join): Update below comments.
	(c_ts18661_flt_eval_method): Set excess_precision_type to
	EXCESS_PRECISION_TYPE_FLOAT16 when -fexcess-precision=16.
	* c-cppbuiltin.c (cpp_atomic_builtins): Update below comments.
	(c_cpp_flt_eval_method_iec_559): Set excess_precision_type to
	EXCESS_PRECISION_TYPE_FLOAT16 when -fexcess-precision=16.

gcc/ChangeLog:

	* common.opt: Support -fexcess-precision=16.
	* config/aarch64/aarch64.c (aarch64_excess_precision): Return
	FLT_EVAL_METHOD_PROMOTE_TO_FLOAT16 when
	EXCESS_PRECISION_TYPE_FLOAT16.
	* config/arm/arm.c (arm_excess_precision): Ditto.
	* config/i386/i386.c (ix86_get_excess_precision): Ditto.
	* config/m68k/m68k.c (m68k_excess_precision): Issue an error
	when EXCESS_PRECISION_TYPE_FLOAT16.
	* config/s390/s390.c (s390_excess_precision): Ditto.
	* coretypes.h (enum excess_precision_type): Add
	EXCESS_PRECISION_TYPE_FLOAT16.
	* doc/tm.texi (TARGET_C_EXCESS_PRECISION): Update documents.
	* doc/tm.texi.in (TARGET_C_EXCESS_PRECISION): Ditto.
	* doc/extend.texi (Half-Precision): Document
	-fexcess-precision=16.
	* flag-types.h (enum excess_precision): Add
	EXCESS_PRECISION_FLOAT16.
	* target.def (excess_precision): Update document.
	* tree.c (excess_precision_type): Set excess_precision_type to
	EXCESS_PRECISION_FLOAT16 when -fexcess-precision=16.

gcc/fortran/ChangeLog:

	* options.c (gfc_post_options): Issue an error for
	-fexcess-precision=16.

gcc/testsuite/ChangeLog:

	* gcc.target/i386/float16-6.c: New test.
	* gcc.target/i386/float16-7.c: New test.