Set -fno-emulated-tls for riscv64

Building for riscv64 fails with link errors on __thread variables.
Set -fno-emulated-tls to fix it.

Bug: 254713216
Test: lunch aosp_riscv64-userdebug && m ALLOW_MISSING_DEPENDENCIES=true ndk_sysroot
Change-Id: I3dca81dfd277d681b6c868a5e8385e3a37335a5f
diff --git a/cc/lto.go b/cc/lto.go
index e068b05..1afa1dd 100644
--- a/cc/lto.go
+++ b/cc/lto.go
@@ -91,7 +91,7 @@
 		return flags
 	}
 
-	// LTO doesn't work on riscv64 yet.
+	// TODO(b/254713216): LTO doesn't work on riscv64 yet.
 	if ctx.Arch().ArchType == android.Riscv64 {
 		return flags
 	}