Use |alignas| for alignment.

MSVC doesn't have stdalign.h and so doesn't support |alignas| in C
code. Define |alignas(x)| as a synonym for |__decltype(align(x))|
instead for it.

This also fixes -Wcast-qual warnings in rsaz_exp.c.

Change-Id: Ifce9031724cb93f5a4aa1f567e7af61b272df9d5
Reviewed-on: https://boringssl-review.googlesource.com/6924
Reviewed-by: Adam Langley <agl@google.com>
Reviewed-by: David Benjamin <davidben@google.com>

crypto/internal.h

diff --git a/crypto/internal.h b/crypto/internal.h
index bf45349..fe4ed73 100644
--- a/crypto/internal.h
+++ b/crypto/internal.h
@@ -112,6 +112,15 @@
 #include <openssl/ex_data.h>
 #include <openssl/thread.h>
 
+#if defined(_MSC_VER)
+#if !defined(__cplusplus) || _MSC_VER < 1900
+#define alignas(x) __declspec(align(x))
+#define alignof __alignof
+#endif
+#else
+#include <stdalign.h>
+#endif
+
 #if defined(OPENSSL_NO_THREADS)
 #elif defined(OPENSSL_WINDOWS)
 #pragma warning(push, 3)