FreeBSD_HEAD_i386 - Build #4516 - Fixed

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FreeBSD_HEAD_i386 - Build #4516 - Fixed:

Build information: https://jenkins.FreeBSD.org/job/FreeBSD_HEAD_i386/4516/
Full change log: https://jenkins.FreeBSD.org/job/FreeBSD_HEAD_i386/4516/changes
Full build log: https://jenkins.FreeBSD.org/job/FreeBSD_HEAD_i386/4516/console

Change summaries:

310793 by avos:
rtwn: silence compiler warning (-Wmaybe-uninitialized).

Reported by:	adrian

310792 by dim:
Revert r310775 for now, until we can figure out why it does not seem to
work properly when cross-building.  Sorry for the breakage.

310791 by gonzo:
[qemu] Fix VERSATILEPB kernel boot in QEMU broken by r300968

QEMU does not implement hardware debug registers so when
dbg_monitor_is_enabled is called kernel receives "invalid instruction"
exception. QEMU implements only DIDR register and on read returns all
zeroes to indicate that it doesn't support other registers. Real
hardware has Version bits set.

MFC after:	1 week

310790 by kan:
Use TARGET_ARCH instead of MACHINE_ARCH for MIPS kernel

MACHINE_ARCH is overwritten by config file and will not
contain -hf suffix, so uname -p reported by kernel will
be wrong.

310789 by kan:
Use compiler driver to build relocatable object

This works better with external toolchains where LD
will not necessarily defailt to emulation we want.
Compiler driver knows better.