Add emulated TLS support

Currently LLVM uses emutls by default
for some targets (such as android, openbsd),
but rust does not use it, because `has_thread_local` is false.

This commit has some changes to allow users to enable emutls:

1. add `-Zhas-thread-local` flag to specify
    that std uses `#[thread_local]` instead of pthread key.
2. when using emutls, decorate symbol names
    to find thread local symbol correctly.
3. change `-Zforce-emulated-tls` to `-Ztls-model=emulated`
    to explicitly specify whether to generate emutls.

compiler/rustc_codegen_llvm/src/llvm/ffi.rs

@@ -2159,7 +2159,7 @@ extern "C" {
         SplitDwarfFile: *const c_char,
         OutputObjFile: *const c_char,
         DebugInfoCompression: *const c_char,
-        ForceEmulatedTls: bool,
+        UseEmulatedTls: bool,
         ArgsCstrBuff: *const c_char,
         ArgsCstrBuffLen: usize,
     ) -> *mut TargetMachine;