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/x86_linux_host.go

@@ -103,6 +103,14 @@ func (t *toolchainLinuxX86) Name() string {
 	return "x86"
 }
 
+func (toolchainLinuxX86) LibclangRuntimeLibraryArch() string {
+	return "i386"
+}
+
+func (toolchainLinuxX8664) LibclangRuntimeLibraryArch() string {
+	return "x86_64"
+}
+
 func (t *toolchainLinuxX86) RustTriple() string {
 	return "i686-unknown-linux-gnu"
 }