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/makevars.go b/cc/makevars.go
index 56698f2..23814c3 100644
--- a/cc/makevars.go
+++ b/cc/makevars.go
@@ -172,7 +172,8 @@
 		}, " "))
 
 		if target.Os.Class == android.Device {
-			ctx.Strict(secondPrefix+"ADDRESS_SANITIZER_RUNTIME_LIBRARY", strings.TrimSuffix(toolchain.AddressSanitizerRuntimeLibrary(), ".so"))
+			ctx.Strict(secondPrefix+"ADDRESS_SANITIZER_RUNTIME_LIBRARY", strings.TrimSuffix(config.AddressSanitizerRuntimeLibrary(toolchain), ".so"))
+			ctx.Strict(secondPrefix+"UBSAN_RUNTIME_LIBRARY", strings.TrimSuffix(config.UndefinedBehaviorSanitizerRuntimeLibrary(toolchain), ".so"))
 		}
 
 		// This is used by external/gentoo/...