testsuite: guard usage of _Float16 in analyzer test [PR104150]

gcc/testsuite/ChangeLog:
	PR analyzer/104150
	* gcc.dg/analyzer/pr104089.c: Add "dg-add-options float16"
	and "dg-require-effective-target float16" directives.

Signed-off-by: David Malcolm <dmalcolm@redhat.com>