Undo r230021 again, further shrinking the diff against upstream.

This revision worked around an endless recursion when compiling clzdi2.c
and ctzdi2.c with gcc, upstream landed a different workaround for this
in https://reviews.llvm.org/rL324593, which is effective enough.

Noticed by: jrtc27

(cherry picked from commit 876f11703dabed6f07d9d7352ceb996c5ffc407f)