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/config.go

@@ -40,6 +40,7 @@ const productVariablesFileName = "soong.variables"
 type FileConfigurableOptions struct {
 	Mega_device *bool `json:",omitempty"`
 	Ndk_abis    *bool `json:",omitempty"`
+	Host_bionic *bool `json:",omitempty"`
 }
 
 func (f *FileConfigurableOptions) SetDefaultConfig() {