Remove USE_CROSS_COMPILE support

[jperkin]

Support for cross-compilation adds a huge amount of additional complexity for all systems, especially in mk/ where we already struggle to keep things readable and maintainable, when in reality it's probably only useful to a very limited number of people running on NetBSD.

Back when I wrote large invasive patchsets such as the multiarch branch I always kept it out of the main pkgsrc branch as the vast majority of people will not be interested, and additional complexity comes with significant cost to everyone.

I'd say that if people want it they should use pkgsrc where it will (I assume) continue to be actively developed.