fe_operations: error on CURVE25519_SMALL/ED25519_SMALL + x64 asm

The small (reduced-C) curve25519/ed25519 code and the Intel x64 assembly
both define the same fe_/sc_/curve25519 symbols. A user_settings.h that
keeps CURVE25519_SMALL/ED25519_SMALL while USE_INTEL_SPEEDUP selects the
x64 assembly (as happens with --enable-usersettings, which does not run
configure's implementation selection) links with duplicate symbols that
are hard to diagnose. Detect the incompatible combination at compile time
via a clear #error keyed on CURVED25519_X64 (the single source of truth
for 'x64 25519 asm active'), so it fails fast instead. NO_CURVED25519_X64
remains the escape hatch to keep the small implementation.

Author: danielinux

wolfssl/wolfcrypt/fe_operations.h

@@ -40,6 +40,20 @@
     #define CURVED25519_ASM_64BIT
     #define CURVED25519_ASM
 #endif
+
+/* The small (reduced-C) curve25519/ed25519 code and the Intel x64 assembly
+ * both provide the same fe_, sc_ and curve25519 symbols, so selecting both
+ * (for example a user_settings.h that keeps CURVE25519_SMALL/ED25519_SMALL
+ * while USE_INTEL_SPEEDUP enables the x64 assembly) produces duplicate-symbol
+ * link errors that are hard to diagnose. Detect the incompatible combination
+ * at compile time with a clear message instead. To keep the small
+ * implementation define NO_CURVED25519_X64; to use the assembly drop
+ * CURVE25519_SMALL / ED25519_SMALL. */
+#if defined(CURVED25519_X64) && \
+    (defined(CURVE25519_SMALL) || defined(ED25519_SMALL))
+    #error "CURVE25519_SMALL/ED25519_SMALL are incompatible with the Intel x64 curve25519/ed25519 assembly (CURVED25519_X64); define NO_CURVED25519_X64 to keep the small implementation, or remove the SMALL settings to use the assembly"
+#endif
+
 #if defined(WOLFSSL_ARMASM)
     #ifdef __aarch64__
         #define CURVED25519_ASM_64BIT