If we're using Clang as the compiler, use it as the assembler too.

Should fix issue reported by Floffy on IRC.

src/build/Common.gmake

@@ -170,9 +170,11 @@ CCPRE	?= ${shell if [ `echo __clang__ | $(CC) -E - | grep -v '^\#'` != __clang__
 ifeq ($(CCPRE),clang)
  ifeq ($(CC),cc)
   CXX	:= c++
+  AS	:= cc
  else
   ifeq ($(CC),clang)
    CXX	:= clang++
+   AS	:= clang
   endif
  endif
 endif