[lit] Raise the default soft process limit when possible

It is common to have a default soft process limit, at least on some families of
Linux distributions, of 1024. This is normally more than enough, but if you
have many cores, and you're running tests that create many threads, this can
become a problem. My POWER7 development machine has 48 cores, and when running
the lld regression tests, which often want to create up to 48 threads, I run
into problems. lit, by default, will want to run 48 tests in parallel, and
48*48 < 1024, and so many tests fail like this:

terminate called after throwing an instance of 'std::system_error'

what():  Resource temporarily unavailable
or lit fails like this when launching a test:

OSError: [Errno 11] Resource temporarily unavailable

lit can easily detect this situation and attempt to repair it before launching
tests (by raising the soft process limit to something that will allow ncpus^2
threads to be created), and should do so to prevent spurious test failures.

This is the follow-up to this thread:
http://lists.llvm.org/pipermail/llvm-dev/2015-October/090942.html

llvm-svn: 249161