aarch64: Add +xs flag for existing instructions

Additionally, change FEAT_XS tlbi variants to be gated on "+xs" instead of
"+d128".  This is an incremental improvement; there are still some FEAT_XS tlbi
variants that are gated incorrectly or missing entirely.

include/opcode/aarch64.h

@@ -141,6 +141,8 @@ enum aarch64_feature_bit {
   AARCH64_FEATURE_MEMTAG,
   /* Transactional Memory Extension.  */
   AARCH64_FEATURE_TME,
+  /* XS memory attribute.  */
+  AARCH64_FEATURE_XS,
   /* WFx instructions with timeout.  */
   AARCH64_FEATURE_WFXT,
   /* Standardization of memory operations.  */
@@ -273,6 +275,7 @@ enum aarch64_feature_bit {
 					 | AARCH64_FEATBIT (X, BFLOAT16) \
 					 | AARCH64_FEATBIT (X, I8MM))
 #define AARCH64_ARCH_V8_7A_FEATURES(X)	(AARCH64_FEATBIT (X, V8_7A)	\
+					 | AARCH64_FEATBIT (X, XS)      \
 					 | AARCH64_FEATBIT (X, WFXT)    \
 					 | AARCH64_FEATBIT (X, LS64))
 #define AARCH64_ARCH_V8_8A_FEATURES(X)	(AARCH64_FEATBIT (X, V8_8A)	\