Add a note on architecture requirements.

The s390x patches keep on coming.

Change-Id: I6d7f79e5ee7c8fcfe6b2e8e549b18ee686b4392b
Reviewed-on: https://boringssl-review.googlesource.com/18564
Reviewed-by: David Benjamin <davidben@google.com>
Commit-Queue: David Benjamin <davidben@google.com>
CQ-Verified: CQ bot account: commit-bot@chromium.org <commit-bot@chromium.org>

include/openssl/base.h

@@ -107,6 +107,10 @@ extern "C" {
 #elif defined(__myriad2__)
 #define OPENSSL_32_BIT
 #else
+/* Note BoringSSL only supports standard 32-bit and 64-bit two's-complement,
+ * little-endian architectures. Functions will not produce the correct answer
+ * on other systems. Run the crypto_test binary, notably
+ * crypto/compiler_test.cc, before adding a new architecture. */
 #error "Unknown target CPU"
 #endif