Add Clang Asflags for mips

Like make, it needs -fPIC to be added to compile successfully.

Change-Id: I1d6f1388c42a84f331bbce1ecd669669441d986e
diff --git a/cc/toolchain.go b/cc/toolchain.go
index ac51114..9f6ad2e 100644
--- a/cc/toolchain.go
+++ b/cc/toolchain.go
@@ -60,6 +60,7 @@
 	ClangSupported() bool
 	ClangTriple() string
 	ToolchainClangCflags() string
+	ClangAsflags() string
 	ClangCflags() string
 	ClangCppflags() string
 	ClangLdflags() string
@@ -112,6 +113,10 @@
 	return ""
 }
 
+func (toolchainBase) ClangAsflags() string {
+	return ""
+}
+
 type toolchain64Bit struct {
 	toolchainBase
 }