Attempt to work around issues with xend being executed unconditionally

Attempt to work around issues with xend being executed unconditionally
when a pthread_rwlock is used in an invalid way.

Recent glibcs use transactional memory instructions to do lock ellision
but will sometimes, when locks are used in an invalid way, may calls to
xend on systems which don't support it, on the grounds that the program
is invalid anyway.

So we try and catch and ignore the resulting SIGILL in our tests that
deliberately work with invalid locks.

git-svn-id: svn://svn.valgrind.org/valgrind/trunk@15565