Create toolchainBionic for the various bionic-based toolchains to inherit from

Host bionic and each of the device bionic architectures will share some
new toolchain functionality in common, create a toolchainBionic and embed
it into the toolchains.  Use it to implement Bionic() once and change
the default implementation from true to false.

Test: go test ./cc/...
Test: m checkbuild
Change-Id: Ia34b80d9691edc4ab1cbdbd005d1ffc950d0881c

cc/config/x86_linux_bionic_host.go

@@ -76,6 +76,7 @@ func init() {
 
 type toolchainLinuxBionic struct {
 	toolchain64Bit
+	toolchainBionic
 }
 
 func (t *toolchainLinuxBionic) Name() string {
@@ -133,10 +134,6 @@ func (t *toolchainLinuxBionic) AvailableLibraries() []string {
 	return nil
 }
 
-func (t *toolchainLinuxBionic) Bionic() bool {
-	return true
-}
-
 func (toolchainLinuxBionic) LibclangRuntimeLibraryArch() string {
 	return "x86_64"
 }