selftests/powerpc: Fix copyloops build since Power4 assembler change

The recent commit 15a3204d ("powerpc/64s: Set assembler machine
type to POWER4") set the machine type in our ASFLAGS when building the
kernel, and removed some ".machine power4" directives from various asm
files.

This broke the selftests build on old toolchains (that don't assume
Power4), because we build the kernel source files into the selftests
using different ASFLAGS.

The fix is simply to add -mpower4 to the selftest ASFLAGS as well.

Fixes: 15a3204d

 ("powerpc/64s: Set assembler machine type to POWER4")
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>