i386: Allow round for HFmode in optab hook [PR127114]

The match.pd narrowing of (_Float16) round ((double) x) to a
native HF round query direct_internal_fn_supported_p with
OPTIMIZE_FOR_BOTH, so align the backend optab_supported_p hook
like the ceil/floor/btrunc did to avoid enforced extend/truncate
at vectorization.

gcc/ChangeLog:

	PR target/127114
	* config/i386/i386.cc (ix86_optab_supported_p): Return true for
	round_optab in HFmode.

gcc/testsuite/ChangeLog:

	PR target/127114
	* gcc.target/i386/pr127114.c: New test.