Work around LLVM PR30879, which is about a bad interaction between X86
Call Frame Optimization on i386 and libunwind, by disallowing the
optimization for i386-freebsd12.

This should fix some instances of broken exception handling when frame
pointers are omitted, in particular some unittests run during the build
of editors/libreoffice.

This hack will be removed as soon as upstream has implemented a more
permanent fix for this problem.

Upstream PR: https://llvm.org/bugs/show_bug.cgi?id=30879
Reviewed by: emaste
PR: 212343