selftests/powerpc: Allow the tm-syscall test to build with old headers

When building against older kernel headers, currently the tm-syscall
test fails to build because PPC_FEATURE2_HTM_NOSC is not defined.

Tweak the test so that if PPC_FEATURE2_HTM_NOSC is not defined it still
builds, but prints a warning at run time and marks the test as skipped.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>