Bug 1122931 - Don't overwrite the AS variable in toolchain.m4; r=glandium

It looks like overwriting AS here is not intentional.  Before this patch,
it is impossible to override AS through mozconfig for anything that runs
past this stage in configure.

build/autoconf/toolchain.m4

@@ -172,7 +172,7 @@ AC_PROG_CXX
 
 AC_CHECK_PROGS(RANLIB, "${target_alias}-ranlib" "${target}-ranlib", :)
 AC_CHECK_PROGS(AR, "${target_alias}-ar" "${target}-ar", :)
-MOZ_PATH_PROGS(AS, "${target_alias}-as" "${target}-as", :)
+AC_CHECK_PROGS(AS, "${target_alias}-as" "${target}-as", :)
 AC_CHECK_PROGS(LD, "${target_alias}-ld" "${target}-ld", :)
 AC_CHECK_PROGS(STRIP, "${target_alias}-strip" "${target}-strip", :)
 AC_CHECK_PROGS(WINDRES, "${target_alias}-windres" "${target}-windres", :)