define builtins for "shared" avxneconvert-avx512bf16vl builtins.

This should fix incorrect error when call those builtin with
-mavxneconvert and w/o -mavx512bf16 -mavx512vl.

gcc/ChangeLog:

	* config/i386/i386-builtins.cc (def_builtin): Handle "shared"
	avx512bf16vl-avxneconvert builtins.

gcc/testsuite/ChangeLog:

	* gcc.target/i386/avxneconvert-1.c: New test.