D152023 made ubsan consider __builtin_clz of 0 undefined regardless of
the target. This ensures portability and matches gcc.
This causes the ACLE intrinsics to also be considered to also be
considered to be undefined for 0 since they used the generic builtins
as their implementation.
This patch adds builtins for ARM that ubsan doesn't know about to make
the behavior defined for 0. Alternatively, I could have added a zero
check to the intrinsics, but the dedicated builtin will give better -O0