The default was erroneously changed, causing P3 builds to have smaller
caches, so switch it back. The RVFI-DII builds override this with a TEST
configuration anyway now.
This one has the advantage of being able to be called with the same
flags as bsc, rather than needing to pass things through special
environment variables. As a result, revert all our changes to dealing
with BSC_COMPILATION_FLAGS (some of this diff therefore looks strangely
formatted, but it's to match upstream verbatim, and should be left that
way to minimise diffs and avoid conflicts).
