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/context.rs

@@ -120,6 +120,7 @@ fn to_llvm_tls_model(tls_model: TlsModel) -> llvm::ThreadLocalMode {
         TlsModel::LocalDynamic => llvm::ThreadLocalMode::LocalDynamic,
         TlsModel::InitialExec => llvm::ThreadLocalMode::InitialExec,
         TlsModel::LocalExec => llvm::ThreadLocalMode::LocalExec,
+        TlsModel::Emulated => llvm::ThreadLocalMode::GeneralDynamic,
     }
 }