build: i18n: `icutrim.py` needs to support BIG-endian machines.

I am tracking some other related ppc/aix issues in https://github.com/srl295/node/issues/7
and this commit is itself a port of https://github.com/srl295/node/commit/ba8ab91bc4762ade646474276c4a4b8cdaf83115

This is itself to support joyent/node/#7676
Update: this is about *big* endian support, of course.