Add a toolchain target spec for AArch64

targets/aarch64-unknown-none.json

+{
+  "llvm-target": "aarch64-unknown-none",
+  "target-endian": "little",
+  "target-pointer-width": "64",
+  "target-c-int-width": "32",
+  "data-layout": "e-m:e-i8:8:32-i16:16:32-i64:64-i128:128-n32:64-S128",
+  "arch": "aarch64",
+  "os": "none",
+  "env": "",
+  "vendor": "unknown",
+  "linker-flavor": "gcc",
+  "target-family": "redox",
+  "pre-link-args": ["-m64", "-nostdlib", "-static"],
+  "features": "+a53,+strict-align,-fp-armv8",
+  "dynamic-linking": false,
+  "executables": false,
+  "relocation-model": "pic",
+  "code-model": "large",
+  "disable-redzone": true,
+  "eliminate-frame-pointer": false,
+  "exe-suffix": "",
+  "has-rpath": false,
+  "no-compiler-rt": true,
+  "no-default-libraries": true,
+  "position-independent-executables": false,
+  "has-elf-tls": true
+}