Wrap MSVC-only warning pragmas in a macro.

There's a __pragma expression which allows this. Android builds us Windows with
MinGW for some reason, so we actually do have to tolerate non-MSVC-compatible
Windows compilers. (Clang for Windows is much more sensible than MinGW and
intentionally mimicks MSVC.)

MinGW doesn't understand MSVC's pragmas and warns a lot. #pragma warning is
safe to suppress, so wrap those to shush them. This also lets us do away with a
few ifdefs.

Change-Id: I1f5a8bec4940d4b2d947c4c1cc9341bc15ec4972
Reviewed-on: https://boringssl-review.googlesource.com/8236
Reviewed-by: Adam Langley <agl@google.com>

crypto/cpu-intel.c

diff --git a/crypto/cpu-intel.c b/crypto/cpu-intel.c
index 431e1e1..f2e0c4c 100644
--- a/crypto/cpu-intel.c
+++ b/crypto/cpu-intel.c
@@ -69,10 +69,10 @@
 #include <string.h>
 
 #if defined(OPENSSL_WINDOWS)
-#pragma warning(push, 3)
+OPENSSL_MSVC_PRAGMA(warning(push, 3))
 #include <immintrin.h>
 #include <intrin.h>
-#pragma warning(pop)
+OPENSSL_MSVC_PRAGMA(warning(pop))
 #endif
 
 #include "internal.h"