arch: fix SIMDE_ARCH_POWER_ALTIVEC_CHECK to include AltiVec check
Previously we were just checking for POWER and assuming AltiVec, which
worked fine on POWER7+, but on POWER6 AltiVec was optional.
#if defined(__ALTIVEC__)
# define SIMDE_ARCH_POWER_ALTIVEC SIMDE_ARCH_POWER
-#endif
-#if defined(SIMDE_ARCH_POWER)
#define SIMDE_ARCH_POWER_ALTIVEC_CHECK(version) ((version) <= SIMDE_ARCH_POWER)
#else
#define SIMDE_ARCH_POWER_ALTIVEC_CHECK(version) (0)
