[Backtracing] Make sure we print the program counter on ARM64 linux.

On Linux, the right macro for detecting ARM64 is `__aarch64__`, not
`__arm64__`.

rdar://118379429

Author: al45tair

stdlib/public/runtime/CrashHandlerLinux.cpp

@@ -240,7 +240,7 @@ handle_fatal_signal(int signum,
   pc = (void *)(ctx->uc_mcontext.gregs[REG_RIP]);
 #elif defined(__i386__)
   pc = (void *)(ctx->uc_mcontext.gregs[REG_EIP]);
-#elif defined(__arm64__)
+#elif defined(__arm64__) || defined(__aarch64__)
   pc = (void *)(ctx->uc_mcontext.pc);
 #elif defined(__arm__)
   pc = (void *)(ctx->uc_mcontext.gprs[15]);