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
diff --git a/cc/config/x86_device.go b/cc/config/x86_device.go
index 032b1f0..4667caa 100644
--- a/cc/config/x86_device.go
+++ b/cc/config/x86_device.go
@@ -251,8 +251,8 @@
 	return "${config.X86Ldflags}"
 }
 
-func (toolchainX86) AddressSanitizerRuntimeLibrary() string {
-	return "libclang_rt.asan-i686-android.so"
+func (toolchainX86) SanitizerRuntimeLibraryArch() string {
+	return "i686"
 }
 
 func x86ToolchainFactory(arch android.Arch) Toolchain {