powerpc: add powerpc64 and powerpc64le support

This enables powerpc64 and powerpc64le. Currently, le needs at least
glibc 2.19 and gcc 4.9.0. For gdb, 7.7.1 works (added in an earlier
patch).

[Peter: also disallow gcc 4.8 for ppc64le]
Signed-off-by: Cody P Schafer <cody@linux.vnet.ibm.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>