Add emulation for legacy x86 task switching mechanism.

FreeBSD/i386 uses task switching to handle double fault exceptions and this
change enables that to work.

Reported by:	glebius
Enable MK_TESTS by default, take 2.

This is a redo of r267424, which was reverted in r267432 because it broke
"make buildworld" from FreeBSD 9.x.  This issue has been resolved and this
change is still "make tinderbox" clean.
Regen after r268778 to replace WITH_TESTS with WITHOUT_TESTS.