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/arm64_device.go

@@ -26,8 +26,8 @@ var (
 	Arm64LinkFlags            = []string{}
 
 	Arm64ArchVariantRustFlags = map[string][]string{
-		"armv8-a":  []string{},
-		"armv8-2a": []string{},
+		"armv8-a":          []string{},
+		"armv8-2a":         []string{},
 		"armv8-2a-dotprod": []string{},
 	}
 )
@@ -71,6 +71,10 @@ func (t *toolchainArm64) Supported() bool {
 	return true
 }
 
+func (toolchainArm64) LibclangRuntimeLibraryArch() string {
+	return "aarch64"
+}
+
 func Arm64ToolchainFactory(arch android.Arch) Toolchain {
 	archVariant := arch.ArchVariant
 	if archVariant == "" {