powerpc/64: Allow CONFIG_RELOCATABLE if COMPILE_TEST

This was a hack we added to work around the allmodconfig build breaking, see
commit fb43e847 ("powerpc: Disable RELOCATABLE for COMPILE_TEST with
PPC64").

Since we merged the thin archives support in commit 43c9127d ("powerpc: Add
option to use thin archives") this hasn't been necessary, so remove it.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>