powerpc/Makefile: Remove workaround for gcc versions below 4.9

Commit 6ec4476a

 ("Raise gcc version requirement to 4.9")
made it impossible to build with gcc 4.8 and under.

Remove related workaround.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/a1e552006b8c51f23edd2f6cabdd9a986c631146.1615380184.git.christophe.leroy@csgroup.eu