Add the cc_fuzz target.

Adds the cc_fuzz target via a cc_fuzz module. Also implements the
libclang runtime interface for the x86 toolchain to allow host-built
fuzzers.

Bug: 133261679
Bug: 137398545

Test: Build a fuzzer (with all topic patches), notice that you now
have a host-built fuzzer :)

Change-Id: I7fa069603415f40b3f12a002c253fca6e2aa1988
diff --git a/cc/config/x86_linux_host.go b/cc/config/x86_linux_host.go
index f072f34..f08a379 100644
--- a/cc/config/x86_linux_host.go
+++ b/cc/config/x86_linux_host.go
@@ -233,6 +233,14 @@
 	return "${config.LinuxX8664YasmFlags}"
 }
 
+func (toolchainLinuxX86) LibclangRuntimeLibraryArch() string {
+	return "i686"
+}
+
+func (toolchainLinuxX8664) LibclangRuntimeLibraryArch() string {
+	return "x86_64"
+}
+
 func (t *toolchainLinux) AvailableLibraries() []string {
 	return linuxAvailableLibraries
 }