Copy head (r256279) to stable/10 as part of the 10.0-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

o) Use ABI, not ISA_* options, to determine whether to compile bits if libkern
required for the ABI the kernel is being built for.
XXX This is implemented in a kind-of nasty way that involves including source
files, but it's still an improvement.
o) Retire ISA_* options since they're unused and were always wrong.

Remove the 'machine mips' from DEFAULTS. Put the proper 'machine mips
mipsel' or 'machine mips mipseb' into the config file (with a few 64's
tossed in for good measure). This will let us build the proper
kernels with different worlds as part of make universe.