perf jit: genelf makes assumptions about endian

Commit 9b07e27f ("perf inject: Add jitdump mmap injection support")
incorrectly assumed that PowerPC is big endian only.

Simplify things by consolidating the define of GEN_ELF_ENDIAN and checking
for __BYTE_ORDER == __BIG_ENDIAN.

The PowerPC checks were also incorrect, they do not match what gcc
emits. We should first look for __powerpc64__, then __powerpc__.

Signed-off-by: Anton Blanchard <anton@samba.org>
Acked-by: Michael Ellerman <mpe@ellerman.id.au>
Cc: Carl Love <cel@us.ibm.com>
Cc: Stephane Eranian <eranian@google.com>
Cc: Sukadev Bhattiprolu <sukadev@linux.vnet.ibm.com>
Cc: linuxppc-dev@lists.ozlabs.org
Fixes: 9b07e27f ("perf inject: Add jitdump mmap injection support")
Link: http://lkml.kernel.org/r/20160329175944.33a211cc@kryten


Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>