Add new sysctl, kern.supported_abis, containing the list of FreeBSD
MACHINE_ARCH values whose binaries this kernel can run. This patch provides
a feature requested for implementing pkgng ABI identifiers in a robust
way.

The list is designed to indicate whether, say, an i386 package can be run on
the current system. If kern.supported_abis contains "i386", then the answer
is yes. Otherwise, the answer is no.

At the moment, this only supports MACHINE_ARCH and MACHINE_ARCH32. As we
gain support for more interesting combinations, this needs to become more
flexible, possibily through the sysent framework, along with the
hw.machine_arch emulation immediately preceding this code in kern_mib.c.

Reviewed by:	imp
MFC after:	3 days