Add ubsan_standalone library name to the toolchain.

and export the library name to make. Refactor the code a bit to avoid repeating the library name
multiple times.

Bug: 22033465
Test: Ran external/clang/build.py for aosp-llvm

Change-Id: I25eb3858eb92e1dd493b09524d559802551b2547

cc/config/x86_64_device.go

@@ -232,6 +232,10 @@ func (t *toolchainX86_64) ClangLdflags() string {
 	return "${config.X86_64Ldflags}"
 }
 
+func (toolchainX86_64) SanitizerRuntimeLibraryArch() string {
+	return "x86_64"
+}
+
 func x86_64ToolchainFactory(arch android.Arch) Toolchain {
 	toolchainCflags := []string{
 		"${config.X86_64ToolchainCflags}",