Block esoteric systems.

In particular, only support amd64 for FreeBSD 12+ (-CURRENT),
Nail the coffin of EOL distros: IGNORE builds before 10.3-RELEASE,
and force -mstackrealign on i386 gcc to put an end to i386
instabilities. (for some reason, FreeBSD 12-CURRENT chooses GCC rather
than base clang, and then breaks the stack alignment for SSE2 again,
causing SIGBUS[1].)

[1] http://beefy11.nyi.freebsd.org/data/head-i386-default/p432608_s312909/logs/rawtherapee-devel-5.0.log

Reported by: pkg-fallout@ [1]