testsuite: Fix up gcc.target/i386/pr109137.c testcase [PR109137]

testsuite: Fix up gcc.target/i386/pr109137.c testcase [PR109137]

The testcase has a couple of small problems:
1) had -m32 in dg-options, that should never be done, instead the test
   should be guarded on ia32
2) adds -fPIC unconditionally (that should be guarded on fpic effective
   target)
3) using #include <string.h> for a RA test seems unnecessary, __builtin_memset
   handles it without the header

2023-03-24  Jakub Jelinek  <jakub@redhat.com>

	PR target/109137
	* gcc.target/i386/pr109137.c: Remove -m32 from dg-options, instead
	require ia32 effective target.  Only add -fPIC for fpic effective
	target.  Remove #include <string.h>, use __builtin_memset instead of
	memset.