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
diff --git a/rust/config/x86_64_device.go b/rust/config/x86_64_device.go
index 5f6e85a..94b719f 100644
--- a/rust/config/x86_64_device.go
+++ b/rust/config/x86_64_device.go
@@ -77,6 +77,10 @@
 	return true
 }
 
+func (toolchainX86_64) LibclangRuntimeLibraryArch() string {
+	return "x86_64"
+}
+
 func x86_64ToolchainFactory(arch android.Arch) Toolchain {
 	toolchainRustFlags := []string{
 		"${config.X86_64ToolchainRustFlags}",