Fixing USE_GCC for LibreOffice on i386

[Kevin Zheng]

Hi there,

This issue was discussed quite a bit on this list a little while ago,
and I'd like to figure out what we should do to fix the problem.

In ports/176967, Gabor (pgj@) pointed out that the WITH_GCC knob fixes
LibreOffice by using GCC instead of Clang. This fix works for me.

In ports/178041, the submitter converted the WITH_GCC knob to a
user-configurable option (via `make config`).

Instead, I'm thinking that we add some logic to the Makefile that
requires GCC when ARCH is i386. This way, users won't have to wrestle
with the knob or configurable option. In addition, this let us keep
using Clang on the systems where it actually works.

Ideas/suggestions/comments? If not, I'd like to send in a PR with my
proposed changes.

Thanks,
Kevin Zheng