Fix icu4c calling on x86.

Some of the function pointer types were wrong, and x86 cares if you
say `int` when you meant `int8_t` (because it feels at liberty to
leave the top bits dirty and ignore them, both sides need to agree
which they're dealing with).

Also slightly improve the wcstoimax and wcstoumax tests, since my
investigation started there.

Bug: http://b/33451822
Test: tests pass on x86, arm, and arm64
Change-Id: I553193962f0cb993666f9f8e415990bba5b669e1