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/toolchain.go

@@ -169,7 +169,7 @@ func (toolchainBase) AvailableLibraries() []string {
 }
 
 func (toolchainBase) Bionic() bool {
-	return true
+	return false
 }
 
 func (t toolchainBase) ToolPath() string {