games/lmpc: allow the port to build on more 32-bit architectures

Convert to idiomatic ONLY_FOR_ARCHS and register one more object
file which depends on the generated "udemy.h" header file to fix
parallel builds (-jX).

While here, adjust LICENSE (it is GPLv2 or later) and factor out
installation of documentation into the dedicated helper target.

PR: 260697
Submitted by: Robert Clausecker