Merge bitcoin-core/secp256k1#965: gen_context: Don't use any ASM

aeece44 gen_context: Don't use any ASM (Tim Ruffing)

Pull request description:

  See bitcoin/bitcoin#22441 , we need to wait for the testing results there.

ACKs for top commit:
  sipa:
    utACK aeece44
  jonasnick:
    ACK aeece44

Tree-SHA512: 52ff90f3dedda90124140de1c2c1c065a2f9374930d6b988d35c37f5eeae97f7d557b7ab0cf99d22add5a76ff8a3e06226572e43949e12d1048cb323d1b3d92b

src/gen_context.c

@@ -13,10 +13,11 @@
 /* We can't require the precomputed tables when creating them. */
 #undef USE_ECMULT_STATIC_PRECOMPUTATION
 
-/* In principle we could use external ASM, but this yields only a minor speedup in
+/* In principle we could use ASM, but this yields only a minor speedup in
    build time and it's very complicated. In particular when cross-compiling, we'd
-   need to build the external ASM for the build and the host machine. */
+   need to build the ASM for the build and the host machine. */
 #undef USE_EXTERNAL_ASM
+#undef USE_ASM_X86_64
 
 #include "../include/secp256k1.h"
 #include "assumptions.h"