How do I use custom assembler for clang?

I've compiled clang to use it as a cross compiler for ARM (by configuring it with ./configure --target=armv7l-unknown-linux-gnueabihf ), but when I try to compile any C code, it tries to use /usr/bin/as. I already have binutils compiled for ARM, and they are in a separate directory. How do I direct clang (or llvm) to use the assembler that I specify?