string: Improve generic strchrnul

string: Improve generic strchrnul

New algorithm read the first aligned address and mask off the unwanted
bytes (this strategy is similar to arch-specific  implementations used
on powerpc, sparc, and sh).

The loop now read word-aligned address and check using the has_zero_eq
function.

Checked on x86_64-linux-gnu, i686-linux-gnu, powerpc64-linux-gnu,
and powerpc-linux-gnu by removing the arch-specific assembly
implementation and disabling multi-arch (it covers both LE and BE
for 64 and 32 bits).

Co-authored-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Noah Goldstein <goldstein.w.n@gmail.com>