Since python creates little-endian utf-16 consistently whether
cross-compiling from Linux or compiling natively on macOS, we could
write a small script that essentially replaces iconv. On the other hand,
we're also doing some manual preprocessing on the InfoPlist.strings.in
files, and we might as well use the preprocessor for that.
So, we augment the preprocessor to allow an explicit output encoding
other than utf-8, and use the preprocessor instead of `sed | iconv`.
Differential Revision: https://phabricator.services.mozilla.com/D96013
ab57a0ef9b