[PATCH, rs6000] Update pr88233.c test (pr91799)

[PATCH, rs6000] Update pr88233.c test (pr91799)

Hi,

This is a follow-up fix to clean up pr91799.  Per review of test results,
it appears that the combination of target and dg-require stanzas is
not sufficient to properly limit the test to 64-bit only on darwin.

This adds an additional dg-require clause to limit the test to 64-bit
environments.

Tested on power7 and power8 using assorted variations of
  make -k check-gcc-c "RUNTESTFLAGS=powerpc.exp=pr88233.c
  --target_board=unix/'{-mcpu=power7,-mcpu=power6,-mcpu=power8}''{-m32,-m64}'"

PR target/91799

2021-01-19  Will Schmidt <will_schmidt@vnet.ibm.com>

gcc/testsuite/ChangeLog:
	* gcc.target/powerpc/pr88233.c: Update dg- stanzas.