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
diff --git a/android/androidmk.go b/android/androidmk.go
index 793947e..ec3abe1 100644
--- a/android/androidmk.go
+++ b/android/androidmk.go
@@ -164,6 +164,11 @@
 		return err
 	}
 
+	// Make does not understand LinuxBionic
+	if amod.Os() == LinuxBionic {
+		return nil
+	}
+
 	if data.SubName != "" {
 		name += data.SubName
 	}