and define it to true for __aarch64__ and Win Arm64 + Visual Studio.
Microsoft's compiler (cl.exe) does not define __aarch64__, but relies on
_M_ARM64 & _M_ARM64EC
Bug: b/277254922
Change-Id: I20e4fa07a4031599db69e3d7ba9050345315ef51
this avoids duplicates between these trees and dsp/, e.g., enc/tree.c,
dec/tree.c, making pulling the whole library source tree into one target
possible
BUG=webp:279
Change-Id: I060a614833c7c24ddd37bf641702ae6a5eef1775