[AARCH64] [MC] add memtag as an alias of mte architecture extension

Add memtag as an alis of met architectture extesion to be consistent
with GNU as.

LINK:https://sourceware.org/bugzilla/show_bug.cgi?id=26339

Reviewed By: nickdesaulniers, MaskRay

Differential Revision: https://reviews.llvm.org/D85620

Author: jcai19

llvm/lib/Target/AArch64/AsmParser/AArch64AsmParser.cpp

@@ -2841,6 +2841,7 @@ static const struct Extension {
     {"predres", {AArch64::FeaturePredRes}},
     {"ccdp", {AArch64::FeatureCacheDeepPersist}},
     {"mte", {AArch64::FeatureMTE}},
+    {"memtag", {AArch64::FeatureMTE}},
     {"tlb-rmi", {AArch64::FeatureTLB_RMI}},
     {"pan-rwv", {AArch64::FeaturePAN_RWV}},
     {"ccpp", {AArch64::FeatureCCPP}},

llvm/test/MC/AArch64/directive-arch_extension.s

@@ -48,6 +48,10 @@ dc cvadp, x7
 irg x0, x1
 // CHECK: irg x0, x1
 
+.arch_extension memtag
+irg x0, x1
+// CHECK: irg x0, x1
+
 .arch_extension tlb-rmi
 tlbi vmalle1os
 // CHECK: tlbi vmalle1os