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



LeslieW/protobuf,LeslieW/protobuf,LeslieW/protobuf,LeslieW/protobuf