Update sf_buf.h to be a straight copy of the amd64 version.
Implement exec_setregs. Set the stack, link register and exception
link register.
Start to implement the copy{in,out} and su*/fu* functions. These only
return success as a failure would result in a kernel panic. The
exception handlers need to be updated for the failure case to work.
Return to userland at the end of fork_trampoline. Signal handling is
missing as we don't yet support signals.
Pull out the guts of pmap_enter to pmap_enter_locked and use it to
implement pmap_enter_object. While here also support userland
mappings in pmap_enter_locked.