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

[jcai19]

Summary

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

[MaskRay]

Looks good. However, GNU as does not support .arch_extension mte ... (https://sourceware.org/bugzilla/show_bug.cgi?id=26339#c2 ) @ostannard

[nickdesaulniers]

Thanks @jcai19 !

[jcai19]

Thanks all!

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

const FeatureBitset Features;
     {"crypto", {AArch64::FeatureCrypto}},
     {"fp", {AArch64::FeatureFPARMv8}},
     {"simd", {AArch64::FeatureNEON}},
     {"ras", {AArch64::FeatureRAS}},
     {"lse", {AArch64::FeatureLSE}},
     {"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}},
     {"rcpc", {AArch64::FeatureRCPC}},
     {"sve", {AArch64::FeatureSVE}},
     {"sve2", {AArch64::FeatureSVE2}},
     {"sve2-aes", {AArch64::FeatureSVE2AES}},
     {"sve2-sm4", {AArch64::FeatureSVE2SM4}},

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

 .arch_extension ccdp
 dc cvadp, x7
 // CHECK: dc cvadp, x7
 
 .arch_extension mte
 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
 
 .arch_extension pan-rwv
 at s1e1wp, x2
 // CHECK: at s1e1wp, x2
 
 .arch_extension ccpp
 dc cvap, x7
 // CHECK: dc cvap, x7
 
 .arch_extension rcpc
 ldapr x0, [x1]
 // CHECK: ldapr x0, [x1]