[Bug 261977] lang/gcc12-devel: enable LTO

https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=261977

--- Comment #23 from Matthias Andree <mandree@FreeBSD.org> ---
Re compiler bootstrapping with LTO, it is pointless per se because you build
the first compiler stage with whatever is on the system (which is pretty
reasonable on FreeBSD), then you build the second compiler stage, i. e. the
full compiler with the stage-1-compiler, and in the third stage, you build the
SAME THING again with the stage-2-compiler and compare stage 2 to stage 3, i.
e. check if the self-compiled compiler is the same.

If it weren't for a comparison, we would not need LTO in stages one and two
because they are disposed of, and only the compiler built in stage 3 will be
used. And in essence, stage 3 is all you build if you build the compiler as a
cross-compiler without full bootstrap (you knew that already). So if our base
compilers are good enough, let's just build all GCC as cross, or at least all
Tier-1 and Tier-2.

And my builder runs up to(*) 16 Zen threads (AMD Ryzen 7 1700, 8 cores w/ 2
threads each), but I usually need to take that down to 4-ish or so because else
this deadlocks pretty soon with the 12 or how many GBytes of RAM I pass to it.
FreeBSD 13.0, ZFS-based poudriere, no swap.

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