hppa: Add string-fza.h, string-fzc.h, and string-fzi.h

Use UXOR,SBZ to test for a zero byte within a word.  While we can
get semi-decent code out of asm-goto, we would do slightly better
with a compiler builtin.

For index_zero et al, sequential testing of bytes is less expensive than
any tricks that involve a count-leading-zeros insn that we don't have.

Checked on hppa-linux-gnu.

Reviewed-by: Adhemerval Zanella <adhemerval.zanella@linaro.org>