Add aarch64 architecture to platform_macros.h (patch by Sylvain Defresne)

Both gcc and clang defines the __aarch64__ macro when building
for the 64-bit AArch64 execution state of ARMv8 processors, so
use this to detect the architecture.




git-svn-id: http://protobuf.googlecode.com/svn/trunk@516 630680e5-0e50-0410-840e-4b1c322b438d