AArch64: Enable AArch64 support

This patch completes the support for AArch64. It adds the last few
tweaks that will enable a AArch64 build of bionic.

Please note that though AArch64 uses the new kernel infrastructure,
where the userspace exported headers live in uapi folders, we still
need to include "kernel/common" in the include search path
(take a look at external/kernel-headers/uapi/asm-generic/signal-defs.h).

Also remove the relro linker flags for now as they crash the foundation
model.

Change-Id: Ie2fa8c1056c84dc15fec4469f2d65b8d0c5e712b
Signed-off-by: Serban Constantinescu <serban.constantinescu@arm.com>