external/boringssl: update #define guards for x86_64-gcc.c.

OS X builds with NO_ASM and was getting both generic.c and x86_64-gcc.c.
This change updates the latter so that it's excluded in NO_ASM builds.

This is a reland of 53b609c9, which got lost in the last BoringSSL sync
because I forgot to send it upstream.

Change-Id: I82462e5cd1d24fa96176c89d77cafc1f7ed0a0fd
diff --git a/src/crypto/bn/asm/x86_64-gcc.c b/src/crypto/bn/asm/x86_64-gcc.c
index ac63934..0496b95 100644
--- a/src/crypto/bn/asm/x86_64-gcc.c
+++ b/src/crypto/bn/asm/x86_64-gcc.c
@@ -1,6 +1,6 @@
 #include <openssl/bn.h>
 
-#if defined(OPENSSL_X86_64) && !defined(OPENSSL_WINDOWS)
+#if !defined(OPENSSL_NO_ASM) && defined(OPENSSL_X86_64) && !defined(OPENSSL_WINDOWS)
 
 #include "../internal.h"
 
@@ -596,4 +596,4 @@
   r[7] = c2;
 }
 
-#endif  /* defined(OPENSSL_X86_64) && !defined(OPENSSL_WINDOWS) */
+#endif  /* !NO_ASM && X86_64 && !WINDOWS */