arm64: makefile: fix uname munging when setting ARCH on native machine

By popular demand, arch/aarch64 is now known as arch/arm64. However,
uname -m (and indeed the GNU triplet) still use aarch64 as the machine
string.

This patch fixes native builds of both the kernel and perf tools by
updating the relevant Makefiles to munge the output of uname -m and
set the ARCH variable appropriately.

Cc: <stable@vger.kernel.org>
Signed-off-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>