Add Toolchain.Bionic()

Move some cc ctx.Host() / ctx.Device() checks over to using
ctx.toolchain().Bionic(). There will be more changes, these are just the
obvious ones dealing with host_ldlibs / crt / system libraries.

Bug: 31559095
Test: out/soong/build.ninja is identical
Change-Id: Ibba60483b4ab0e01f6996eb7d883120e4acc1830

cc/config/x86_linux_host.go

@@ -256,6 +256,10 @@ func (t *toolchainLinux) AvailableLibraries() []string {
 	return linuxAvailableLibraries
 }
 
+func (t *toolchainLinux) Bionic() bool {
+	return false
+}
+
 var toolchainLinuxX86Singleton Toolchain = &toolchainLinuxX86{}
 var toolchainLinuxX8664Singleton Toolchain = &toolchainLinuxX8664{}