[testsuite] enable -maltivec like vect_int for signbit-2.c

Explicitly enable altivec if it's supported.  vect_int tests for
powerpc_altivec_ok, but without the explicit option, if altivec is not
enabled by default, we end up expecting vectorization that doesn't
occur.


for  gcc/testsuite/ChangeLog

	* gcc.dg/signbit-2.c: Add -maltivec if supported.

gcc/testsuite/gcc.dg/signbit-2.c

@@ -4,6 +4,7 @@
 /* This test does not work when the truth type does not match vector type.  */
 /* { dg-additional-options "-mno-avx512f" { target { i?86-*-* x86_64-*-* } } } */
 /* { dg-additional-options "-march=armv8-a" { target aarch64_sve } } */
+/* { dg-additional-options "-maltivec" { target powerpc_altivec_ok } } */
 /* { dg-skip-if "no fallback for MVE" { arm_mve } } */
 
 #include <stdint.h>