gzdoom: only build for 64-bit architectures

gzdoom: only build for 64-bit architectures

Only LP64/LLP64 builds are officially supported. Please do not attempt
to build for other platforms; even if the program succeeds in a MAP01
smoke test, there are e.g. known visual artifacts that lead to a bad
user experience.

Still allow build on 32-bit platforms in case BROKEN is selected, so
users can try if it would work on their system as forum claims a fix
had actually been applied long ago.

Link: https://forum.zdoom.org/viewtopic.php?f=7&t=75673
Signed-off-by: Daniel Golle <daniel@makrotopia.org>