Use hermetic ar in llvm mode, just like with gcc.

Before this change, builder-funcs would use the host ar in llvm mode,
and the system ar on OS X doesn't understand all the flags that
builder-funcs writes.  Instead, use the ar binary from the hermetic
toolchain in llvm mode.

Change-Id: Ie65e510aba263f1d29af99a4d48996484ce00992