mld_sys_cap: Unconditionally define all sys_cap enum entries

- This commit include the change from mlkem-native:
  pq-code-package/mlkem-native#1319
- The CBMC proofs for the 3 native aarch64 versions of
  Keccak using SHA-3 instructions are failing as the MLD_SYS_CAP_SHA3
  symbol is not defined on an x86 system.
- This commit fixes this by defining the mld_sys_cap entries unconditional.

- We include same fix here to prevent similar issue.

Signed-off-by: willieyz <willie.zhao@chelpis.com>

Author: willieyz

mldsa/src/sys.h

@@ -216,15 +216,10 @@
 /* System capability enumeration */
 typedef enum
 {
-#if defined(MLD_SYS_X86_64)
-  MLD_SYS_CAP_AVX2
-#elif defined(MLD_SYS_AARCH64)
+  /* x86_64 */
+  MLD_SYS_CAP_AVX2,
+  /* AArch64 */
   MLD_SYS_CAP_SHA3
-#else
-  /* C90 does not allow empty enums, so use a dummy value
-   * for architectures other than AArch64 and x86_64. */
-  MLD_SYS_CAP_DUMMY
-#endif
 } mld_sys_cap;
 
 #if !defined(MLD_CONFIG_CUSTOM_CAPABILITY_FUNC)