[GH #935] honor user-rlimits for returned sysmem_amount (untested)

We need to check for RLIMIT_DATA and then RLIMIT_AS,
first hard, then soft limits.
[GH #935] add t/stress/gc.t

use from cmd-line as ./parrot t/stress/gc_2.pir 1000000
ulimit -Sv 64000
./parrot t/stress/gc_3.pir 1000000
ulimit -Sv unlimited
honor rlimit on darwin also (just RSS, not AS) GH #935

With this change and ulimit -v 640 the GC bugs are easily
reproduced now also on darwin.
Merge branch 'smoke-me/rlimit-gh935'