Don't assume that Fiat assembly is available on Windows.

MinGW defines __GNUC__, but the needed assembly file isn't built on
Windows.

See https://stackoverflow.com/questions/77983104/grpc-for-c-dependency-build-failed-curve25519-64-adx-h16-undefined-reference

Change-Id: Idd1f12a5ab030ddd11e7d6e5a44a112acd6096d0
Reviewed-on: https://boringssl-review.googlesource.com/c/boringssl/+/66189
Commit-Queue: Adam Langley <agl@google.com>
Reviewed-by: David Benjamin <davidben@google.com>

crypto/curve25519/internal.h

diff --git a/crypto/curve25519/internal.h b/crypto/curve25519/internal.h
index 0cd1a12..a40f109 100644
--- a/crypto/curve25519/internal.h
+++ b/crypto/curve25519/internal.h
@@ -32,7 +32,7 @@
 #endif
 
 #if !defined(OPENSSL_NO_ASM) && !defined(OPENSSL_SMALL) && \
-    defined(__GNUC__) && defined(__x86_64__)
+    defined(__GNUC__) && defined(__x86_64__) && !defined(OPENSSL_WINDOWS)
 #define BORINGSSL_FE25519_ADX
 
 // fiat_curve25519_adx_mul is defined in