[gn build] Add host_cpu=arm64 & current_os=linux => aarch64-unknown-linux-gnu

[peterwaller-arm]

Summary

I've been using this triple in development for a while without issues,
it's passing check-llvm and check-clang.

(The above is the commit message, but the build is currently broken since
D114639, I intend to submit this once it's passing again and it's accepted in
review)

[aeubanks]

seems fine to me

llvm/utils/gn/secondary/compiler-rt/test/BUILD.gn

write_cmake_config("lit_common_configured") {
   if (llvm_enable_expensive_checks) {
     values += [ "LLVM_ENABLE_EXPENSIVE_CHECKS_PYBOOL=True" ]
   } else {
     values += [ "LLVM_ENABLE_EXPENSIVE_CHECKS_PYBOOL=False" ]
   }
 
   if (host_cpu == "x64") {
     values += [ "HOST_ARCH=x86_64" ]
+  } else if (host_cpu == "arm64") {
+    values += [ "HOST_ARCH=aarch64" ]
   } else {
     assert(false, "unimplemented host_cpu " + host_cpu)
   }
 
   if (host_os == "mac") {
     values += [ "HOST_OS=Darwin" ]
   } else if (host_os == "linux") {
     values += [ "HOST_OS=Linux" ]

llvm/utils/gn/secondary/llvm/triples.gni

if (current_cpu == "x86") {
   if (current_os == "android") {
     llvm_current_triple = "arm-linux-androideabi29"
   }
 } else if (current_cpu == "arm64") {
   if (current_os == "android") {
     llvm_current_triple = "aarch64-linux-android29"
   } else if (current_os == "ios" || current_os == "mac") {
     llvm_current_triple = "arm64-apple-darwin"
+  } else if (current_os == "linux") {
+    llvm_current_triple = "aarch64-unknown-linux-gnu"
   }
 } else if (current_cpu == "ppc64") {
   if (current_os == "linux") {
     llvm_current_triple = "powerpc64le-unknown-linux-gnu"
   }
 }
 
 if (!defined(llvm_current_triple)) {
   assert(false, "unimplemented cpu/os " + current_cpu + "/" + current_os)
 }
 
 declare_args() {
   # The default target triple.
   llvm_target_triple = llvm_current_triple
 }