Add Rust fuzzing support.

Add a rust_fuzz module which builds a libfuzzer binary that enabes
asan+sancov. This relies on the libfuzzer-sys crate.

Bug: 147140513
Test: Local rust_fuzz example builds, fuzzes with asan+sancov.
Change-Id: I57db3b8d25869791824ccfab768d13b0bb9d42fa

rust/config/arm_device.go

@@ -72,6 +72,10 @@ func (t *toolchainArm) Supported() bool {
 	return true
 }
 
+func (toolchainArm) LibclangRuntimeLibraryArch() string {
+	return "arm"
+}
+
 func ArmToolchainFactory(arch android.Arch) Toolchain {
 	toolchainRustFlags := []string{
 		"${config.ArmToolchainRustFlags}",