Implement most of timer_{create,settime,gettime,getoverrun,delete}
for amd64/linux32. Fix the entirely bogus (untested) version from
r161310 for i386/linux using the same shared code in compat/linux.

It is unclear to me if we could support more clock mappings but
the current set allows me to successfully run commercial
32bit linux software under linuxolator on amd64.

Reviewed by: jhb
Differential Revision: D784
MFC after: 3 days
Sponsored by: DARPA, AFRL