Don't use x86_64-gcc.c with NO_ASM.

Android (on 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.

Change-Id: I1f0e1c5e551eed9c575ce632ec3016fce7ec9d2e
Reviewed-on: https://boringssl-review.googlesource.com/4741
Reviewed-by: Adam Langley <agl@google.com>

crypto/bn/asm/x86_64-gcc.c

diff --git a/crypto/bn/asm/x86_64-gcc.c b/crypto/bn/asm/x86_64-gcc.c
index ac63934..0496b95 100644
--- a/crypto/bn/asm/x86_64-gcc.c
+++ b/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 */