profile: Protect .gcda output with flock

This avoids crashing or corrupting data if multiple concurrent
processes write to the same .gcda file. This is hard to test, since
the previous behaviour was a data race that often worked out, and it
ignores errors in flock to fall back to the old racy behaviour so that
it won't degrade the behaviour on filesystems that don't support
flock.

llvm-svn: 234036