OpenWrt / uClibc patches

These changes enable the generic `--cpu:mips` in the `nim.cfg`. This should help increase popularity and coverage of the future support of MIPS platforms for Nim. @Araq has already mentioned a willingness to **fully support uClibc**, this should be a good addition to that.

This also removes the use of `execvpe` in `osproc` module, a function that does not exist on OpenWrt / uClibc.