Fix 325714  Empty vgcore but RLIMIT_CORE is big enough (too big)

Use ULong rather than UInt for coredump file size.

Tested manually by
  ulimit -c 4294967296
  ./vg-in-place ./gdbserver_tests/gone abort
valgrind creates vgcore.xxxxx unusable by GDB.

With the fix, GDB can read the core dump.

Patch from  Romain Geissler



git-svn-id: svn://svn.valgrind.org/valgrind/trunk@13762 a5019735-40e9-0310-863c-91ae7b9d1cf9