lib/crypto: curve25519-hacl64: Fix older clang KASAN workaround for GCC

Commit 2f13dae ("lib/crypto/curve25519-hacl64: Disable KASAN with
clang-17 and older") inadvertently disabled KASAN in curve25519-hacl64.o
for GCC unconditionally because clang-min-version will always evaluate
to nothing for GCC. Add a check for CONFIG_CC_IS_CLANG to avoid applying
the workaround for GCC, which is only needed for clang-17 and older.

Cc: stable@vger.kernel.org
Fixes: 2f13dae ("lib/crypto/curve25519-hacl64: Disable KASAN with clang-17 and older")
Signed-off-by: Nathan Chancellor <nathan@kernel.org>
Acked-by: Ard Biesheuvel <ardb@kernel.org>
Link: https://lore.kernel.org/r/20251103-curve25519-hacl64-fix-kasan-workaround-v2-1-ab581cbd8035@kernel.org
Signed-off-by: Eric Biggers <ebiggers@kernel.org>

Author: nathanchance

lib/crypto/Makefile

@@ -90,7 +90,7 @@ else
 libcurve25519-$(CONFIG_CRYPTO_LIB_CURVE25519_GENERIC) += curve25519-fiat32.o
 endif
 # clang versions prior to 18 may blow out the stack with KASAN
-ifeq ($(call clang-min-version, 180000),)
+ifeq ($(CONFIG_CC_IS_CLANG)_$(call clang-min-version, 180000),y_)
 KASAN_SANITIZE_curve25519-hacl64.o := n
 endif