Work around LLVM bug where TLS not emulated in LTO mode

In LTO mode the backend constructs a target exclusively from command line flags,
without opportunity to accept code gen arguments from Clang. Clang does not
currently pass the -emulated-tls parameter for Android, even though it defaults
to emulated TLS for Android in non-LTO builds. Until this is fixed upstream, we
can explicitly specify that the LTO output for Android should use emulated TLS.

Test: build libandroid with LTO, verified that it uses __emutls_get_address
rather than __tls_get_addr.

Change-Id: I18acac41aac885fc6635fbd55f96ba7c845eb5e7

cc/lto.go

@@ -64,6 +64,11 @@ func (lto *lto) flags(ctx BaseModuleContext, flags Flags) Flags {
 	if Bool(lto.Properties.Lto) {
 		flags.CFlags = append(flags.CFlags, "-flto")
 		flags.LdFlags = append(flags.LdFlags, "-flto")
+		if ctx.Device() {
+			// Work around bug in Clang that doesn't pass correct emulated
+			// TLS option to target
+			flags.LdFlags = append(flags.LdFlags, "-Wl,-plugin-opt,-emulated-tls")
+		}
 		flags.ArFlags = append(flags.ArFlags, " --plugin ${config.LLVMGoldPlugin}")
 	}
 	return flags