testsuite: Fix up pr110915* tests on i686-linux [PR110915]
These tests FAIL on i686-linux, with
.../gcc/testsuite/gcc.dg/pr110915-1.c:8:1: warning: MMX vector return without MMX enabled changes the ABI [-Wpsabi]
.../gcc/testsuite/gcc.dg/pr110915-1.c:7:15: warning: MMX vector argument without MMX enabled changes the ABI [-Wpsabi]
excess warnings. I've added -Wno-psabi to quiet that up, plus I think
it is undesirable to define macros like vector before including C library
headers in case the header would use that identifier in non-obfuscated
form somewhere.