selftests/powerpc/pmu: Link ebb tests with -no-pie

When running the ebb tests after building on a ppc64le Ubuntu machine:

 $ pmu/ebb/reg_access_test: error while loading shared libraries:
 R_PPC64_ADDR16_HI reloc at 0x000000013a965130 for symbol `' out of
 range

This is because the Ubuntu toolchain builds has PIE enabled by default.
Change it to be always off instead.

Signed-off-by: Joel Stanley <joel@jms.id.au>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>