powerpc: check for support for -Wa,-m{power4,any}

LLVM's integrated assembler does not like either -Wa,-mpower4
or -Wa,-many. So just don't pass them if they're not supported.

Signed-off-by: Daniel Axtens <dja@axtens.net>
Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20211221055904.555763-3-aik@ozlabs.ru