[libunwind] Clean up __arm64__

`__aarch64__` is defined for the target (since the beginning of arm64 support: clang 3.5).
`__arm64__` is only defined for the Darwin OS on AArch64.

`defined(__aarch64__) || defined(__arm64__)` can be simplied as `defined(__aarch64__)`

Darwin AArch64 uses %% as the assembly separator (see AArch64MCAsmInfo.cpp).
Make the intention explicit in src/assembly.h

With this change, the libunwind code base has no reference of `__arm64__`/`__arm64`.

Reviewed By: #libunwind, ldionne, mstorsjo

Differential Revision: https://reviews.llvm.org/D77829

Cr-Mirrored-From: https://chromium.googlesource.com/external/github.com/llvm/llvm-project
Cr-Mirrored-Commit: f3d460ce5874e4dd12b8cf8b227de4cec43a685c
diff --git a/src/Unwind_AppleExtras.cpp b/src/Unwind_AppleExtras.cpp
index 248d995..5363039 100644
--- a/src/Unwind_AppleExtras.cpp
+++ b/src/Unwind_AppleExtras.cpp
@@ -56,7 +56,7 @@
           __attribute__((visibility("default"))) const char sym##_tmp42 = 0; \
        extern const char sym##_tmp43 __asm("$ld$hide$os4.3$_" #sym ); \
           __attribute__((visibility("default"))) const char sym##_tmp43 = 0;
-#elif defined(__arm64__)
+#elif defined(__aarch64__)
   #define NOT_HERE_BEFORE_10_6(sym)
   #define NEVER_HERE(sym)
 #else