Supported cross-compile Linux32/64 when building on a different platform.

Target platform could be overridden in build.sh, see "PLATFORM=linux..." line.

Genmake:
- added "unix32" and "unix64" PLATFORM values, added "linux32" and "linux64" TARGET values, used to explicitly set desired platform bitness
- added support for LINKFLAGS when compiling with gcc (worked only with Visual Studio before)