rust: Enable ELF TLS by default in platform Rust

Upstream currently uses pthread keys to emulate TLS on Android because
it historically did not have it, and is going to move to LLVM emulated
TLS, which is needed to target L. Since we know the platform is Q+, we
can just use the new features.

Test: Treehugger
Change-Id: I7c7747d89cba736377cfeb4080573a89b801e8fe

rust/config/global.go

@@ -72,6 +72,8 @@ var (
 		"-C panic=abort",
 		// Generate additional debug info for AutoFDO
 		"-Z debug-info-for-profiling",
+		// Android has ELF TLS on platform
+		"-Z tls-model=global-dynamic",
 	}
 
 	deviceGlobalLinkFlags = []string{