lang/swift510: fix build on low RAM, ccache support

The port did not pass down MAKE_JOBS_NUMBER to swift's build
scripts, and Swiwt/LLVM are C++ ports and memory hungry.

Also, due to lack of CCACHE support, the port cannot pick up
where the OOM killer struck easily in, say poudriere.

Pass down --jobs ${MAKE_JOBS_NUMBER} if MAKE_JOBS_NUMBER is
defined and not empty.

While here, also support CCACHE builds so we can actually restart
in case the MAKE_JOBS_NUMBER is still too high relative to the
build host's memory.

This let me successfully build in one single attempt
with MAKE_JOBS_NUMBER=5 in /etc/make.conf on a 16 GB computer
that previously failed persistently because the port
attempted to build with all I have, meaning 16 jobs.

While here, support CCACHE build.

Approved by: portmgr@ (blanket just-fix-it)