Fix exit status of expr version of as_func_arith.

Fix exit status of expr version of as_func_arith.

* lib/m4sugar/m4sh.m4 (_AS_VAR_ARITH_PREPARE): Count an exit
status of 1 of expr also as success, to avoid failure if the
computation result is zero.  Fixes test failures with IRIX sh,
where the expr variant of as_func_arith is used.

Signed-off-by: Ralf Wildenhues <Ralf.Wildenhues@gmx.de>