Stylistic tweak lwp/process.o machinery

Rename process.s to process.default.s so that the name "process.s" is free
for use, and switch to using that as the ephemeral file name.  This enables
the use of gcc for ${AS}, despite gcc's ignoring files with extensions that
it does not recognize.

Change-Id: Idc0716547770fe4fc94bc3fa2c223966f3f76c3b
Reviewed-on: http://gerrit.openafs.org/11535
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Reviewed-by: Chas Williams - CONTRACTOR <chas@cmf.nrl.navy.mil>
Reviewed-by: D Brashear <shadow@your-file-system.com>