tests/tcg/ppc64le: Fix compile flags for byte_reverse

tests/tcg/ppc64le: Fix compile flags for byte_reverse

With a host compiler new enough to recognize power10 insns,
CROSS_CC_HAS_POWER10 is true, but we do not supply the -cpu
option to the compiler, resulting in

/tmp/ccAVdYJd.s: Assembler messages:
/tmp/ccAVdYJd.s:49: Error: unrecognized opcode: `brh'
/tmp/ccAVdYJd.s:78: Error: unrecognized opcode: `brw'
/tmp/ccAVdYJd.s:107: Error: unrecognized opcode: `brd'
make[2]: *** [byte_reverse] Error 1

Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Cédric Le Goater <clg@kaod.org>