Fix standalone Win64 build.

generic.c still needs to include generic implementations in Win64.
Those are currently done with inline assembly and won't work on
MSVC.

Change-Id: Ifeb5470872d8c97b2ccffeae6f3ccb5661051de3
Reviewed-on: https://boringssl-review.googlesource.com/2102
Reviewed-by: Adam Langley <agl@google.com>

crypto/bn/asm/x86_64-gcc.c

@@ -56,6 +56,8 @@
  *    machine.
  */
 
+ /* TODO(davidben): Get this file working on Windows x64. */
+
 #undef mul
 #undef mul_add
 

crypto/bn/generic.c

@@ -61,8 +61,13 @@
 #include "internal.h"
 
 
+/* Generic implementations of most operations are needed for:
+ * - Configurations without inline assembly.
+ * - Architectures other than x86 or x86_64.
+ * - Windows x84_64; x86_64-gcc.c does not build on MSVC. */
 #if defined(OPENSSL_NO_ASM) || \
-    (!defined(OPENSSL_X86_64) && !defined(OPENSSL_X86))
+    (!defined(OPENSSL_X86_64) && !defined(OPENSSL_X86)) || \
+    (defined(OPENSSL_X86_64) && defined(OPENSSL_WINDOWS))
 
 #if defined(OPENSSL_WINDOWS)
 #define alloca _alloca