Fix ToolchainLdflags calls

Instead of having the baseCompiler adjust the Ldflags, teach
objectLinker about ToolchainLdflags. And don't assume that
ToolchainLdflags will work for Clang too, add a ToolchainClangLdflags
call.

Change-Id: I5f546e46f32edf3fcf75a44bee2c5ee103bc5de3
diff --git a/cc/toolchain.go b/cc/toolchain.go
index 1f51384..397ec09 100644
--- a/cc/toolchain.go
+++ b/cc/toolchain.go
@@ -60,6 +60,7 @@
 	ClangSupported() bool
 	ClangTriple() string
 	ToolchainClangCflags() string
+	ToolchainClangLdflags() string
 	ClangAsflags() string
 	ClangCflags() string
 	ClangCppflags() string
@@ -104,6 +105,10 @@
 	return ""
 }
 
+func (toolchainBase) ToolchainClangLdflags() string {
+	return ""
+}
+
 func (toolchainBase) ClangSupported() bool {
 	return true
 }