[rs6000] [testsuite] Fix test-frame-related.c [PR123129]

The testcase test-frame-related.c fails in 32-bit mode due to
constraints not matching. Use -mpowerpc64 option to ensure that the
testcase works with -m32.

gcc/testsuite:
	PR testsuite/123129
	* gcc.dg/rtl/powerpc/test-frame-related.c: Add -mpowerpc64.

(cherry picked from commit a2eb399ec1de4c33f3e309541f99d543e39291d2)