selftests/powerpc/ptrace: Split CFLAGS better

Currently all ptrace tests are built 64-bit and with TM enabled.

Only the TM tests need TM enabled, so split those out into a separate
variable so that can be specified precisely.

Split the rest of the tests into a variable, and add -m64 to CFLAGS for
those tests, so that in a subsequent patch some tests can be made to
build 32-bit.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20220627140239.2464900-3-mpe@ellerman.id.au