Revise stubs for ARCH_{K8,PPC,ARM}.

* ARCH_K8 and ARCH_ARM now work correctly on MSVC.
* ARCH_PPC now uses the same macro as tcmalloc.

Microsoft documentation:
https://docs.microsoft.com/en-us/cpp/preprocessor/predefined-macros?view=vs-2019

PowerPC documentation:
http://openpowerfoundation.org/wp-content/uploads/resources/leabi/content/dbdoclet.50655243_75216.html

PiperOrigin-RevId: 310160787

Author: pwnall

snappy-stubs-internal.h

@@ -69,19 +69,13 @@
 
 #include "snappy-stubs-public.h"
 
-#if defined(__x86_64__)
-
-// Enable 64-bit optimized versions of some routines.
+// Used to enable 64-bit optimized versions of some routines.
+#if defined(__x86_64__) || defined(_M_X64)
 #define ARCH_K8 1
-
-#elif defined(__ppc64__)
-
+#elif defined(__PPC64__) || defined(__powerpc64__)
 #define ARCH_PPC 1
-
-#elif defined(__aarch64__)
-
+#elif defined(__aarch64__) || defined(_M_ARM64)
 #define ARCH_ARM 1
-
 #endif
 
 // Needed by OS X, among others.