rawtherapee: fix exit SIGABRT, enable OpenMP/i386

Backport 'Explicitly destroy static Glib::RefPtr<>s on exit' from
Hombre57's hidpi-icons branch in GitHub (now merged to dev branch).
Also expose pixbufCache globally and explicitly .clear() it before exit().
Fixes the SIGABRT-on-exit issue.

Remove the wrapper that runs ulimit -c 0 before rawtherapee.

Enable OpenMP on i386 (uses GCC for now).

Shorten option descriptions so they don't get truncated.

Only add -msse2 on i386 and amd64, to fix broken aarch64/powerpc64 builds.

Fix various portlint warnings, removing USE_LDCONFIG.

PR: 205370