Add LinuxBionic toolchain and switch

This adds a toolchain definition for LinuxBionic that only supports
Clang/64-bit. It pulls pieces from the x86_linux_host and x86_64_device
configs, and uses the android clang triple, with some manual overrides.

To enable building this, set your soong.config file to:

 {"Host_bionic": true}

Bug: 31559095
Test: out/soong/{Android,make-vars}-aosp_arm64.mk the same with or
      without host bionic turned on
Test: No change to out/soong/build.ninja before/after this change
Change-Id: Id97dda8bd9aa670c32aed31fbe6aaa8175e70b59

android/arch.go

@@ -781,6 +781,10 @@ func decodeTargetProductVariables(config *config) (map[OsClass][]Target, error)
 		addTarget(BuildOs, *variables.HostSecondaryArch, nil, nil, nil)
 	}
 
+	if config.Host_bionic != nil && *config.Host_bionic {
+		addTarget(LinuxBionic, "x86_64", nil, nil, nil)
+	}
+
 	if variables.CrossHost != nil && *variables.CrossHost != "" {
 		crossHostOs := osByName(*variables.CrossHost)
 		if crossHostOs == NoOsType {