rs6000/testsuite: xfail bswap-brw.c

This testcase does not generate anywhere near optimal code for 32-bit
code.  For p10 it actually now fails this testcase, after the previous
patch.  Let's xfail it.

2022-04-21  Segher Boessenkool  <segher@kernel.crashing.org>

gcc/testsuite/
	PR target/103197
	PR target/102146
	* gcc.target/powerpc/bswap-brw.c: Add xfail on scan-assembler for -m32.