Update linux_bionic configuration

Add a missing toolchain method, don't require modules to opt in to
linux_bionic.

Bug: 31559095
Test: attempt to build host bionic
Change-Id: Ia5c8e2f127b88f56191b7abc389d5329cfdb0720

cc/config/x86_linux_bionic_host.go

@@ -139,6 +139,10 @@ func (t *toolchainLinuxBionic) Bionic() bool {
 	return true
 }
 
+func (toolchainLinuxBionic) LibclangRuntimeLibraryArch() string {
+	return "x86_64"
+}
+
 var toolchainLinuxBionicSingleton Toolchain = &toolchainLinuxBionic{}
 
 func linuxBionicToolchainFactory(arch android.Arch) Toolchain {