gpt and zfs boot blocks are not nearly as size-constrained as boot2
from which they inherited their current optimization and anti-optimization
options. As such the current options do not provide any benefit, but
make debugging of the code much harder.
Also, it has been demonstrated that combination of -mrtd and
-fno-unit-at-a-time may result in mis-compilation of the boot code
with the current base gcc.
Additionally, intermediate assembly file filtering is removed for
zfsboot.
The new boot blocks are all compile- and boot- tested using qemu.
gptzfsboot is tested with real hardware.
Reported by: Peter Jeremy <peterjeremy@acm.org> [miscompilation]
Discussed with: bde, jhb
Tested by: Sebastian Chmielewski <chmielsster@gmail.com> [gptzfsboot]
Approved by: re (kib)
MFC after: 3 weeks
918314631b