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.