Add target spec

x86_64-unknown-redox.json

+{
+    "llvm-target": "x86_64-unknown-redox",
+    "target-endian": "little",
+    "target-pointer-width": "64",
+    "data-layout": "e-m:e-i64:64-f80:128-n8:16:32:64-S128",
+    "arch": "x86_64",
+    "os": "redox",
+    "env": "",
+    "vendor": "unknown",
+    "target-family": "redox",
+    "pre-link-args": ["-m64", "-nostdlib", "-static"],
+    "post-link-args": ["build/userspace/libopenlibm.a"],
+    "features": "-sse3,-ssse3,-sse4.1,-sse4.2,-3dnow,-3dnowa,-avx,-avx2",
+    "dynamic-linking": false,
+    "executables": true,
+    "relocation-model": "static",
+    "code-model": "default",
+    "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
+}