[vxworks] [testsuite] [aarch64] use builtin in pred-not-gen-4.c

[vxworks] [testsuite] [aarch64] use builtin in pred-not-gen-4.c

On vxworks, isunordered is defined as a macro that ultimately calls a
_Fpcomp function, that GCC doesn't recognize as a builtin, so it
can't optimize accordingly.

Use __builtin_isunordered instead to get the desired code for the
test.

for  gcc/testsuite/ChangeLog

	* gcc.target/aarch64/sve/pred-not-gen-4.c: Drop math.h
	include, call builtin.