Try to fix powerpc build, add LIBUNWIND option.

We've had a USES+=compiler:c11 on powerpc64,
see https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=231570
extend and use it on all powerpc* in an attempt to fix PR 242798.

If that doesn't help, add a LIBUNWIND option that makes e2fsck's
signal catcher use libunwind instead of libexecinfo - FreeBSD's
stack unwinder doesn't currently work from signal handlers (#243746),
libunwind does and is supposed to work on powerpc and powerpc64,
among others.

PR: 242798
Reported by: canardo909@gmx.com