lwp: test for working swapcontext() and friends

lwp: test for working swapcontext() and friends

The USE_UCONTEXT was a temporary fix until someone wrote a reasonable
test of the user context swapping functions.  While these are present
on many operating systems, they simply aren't complete.  This test
correctly fails on IRIX, MacOS and some versions of Linux (in particular,
ppc 32-bit binaries on a 64-bit processor).

Change-Id: Ie2f340d661002c8f3c78dddbaf755257cb4ca286
Reviewed-on: http://gerrit.openafs.org/4018
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementia.org>