[GR-46399] Set Xmx when building native images to use a reliable amount of memory

* Avoids using too much memory in CIs with non-isolated parallel runners (macOS).
* It fails early if there is not enough memory available to build the image.
* The value is chosen to be the lowest with still a good build time (not too many GCs):

CE:
Default (= all available memory at start of the build):
10.9s (5.8% of total time) in 81 GCs | Peak RSS: 10.47GB | CPU load: 6.19
[librubyvm:122028] Finished generating 'librubyvm' in 3m 8s.
Xms6g:
8.1s (3.8% of total time) in 135 GCs | Peak RSS: 7.30GB | CPU load: 5.92
[librubyvm:106905] Finished generating 'librubyvm' in 3m 33s.
second run:
8.4s (4.4% of total time) in 139 GCs | Peak RSS: 7.43GB | CPU load: 6.10
[librubyvm:124778] Finished generating 'librubyvm' in 3m 10s.
Xms5g:
9.7s (5.1% of total time) in 157 GCs | Peak RSS: 6.45GB | CPU load: 6.06
[librubyvm:109471] Finished generating 'librubyvm' in 3m 11s.
Xms4g:
31.5s (15.0% of total time) in 197 GCs | Peak RSS: 5.47GB | CPU load: 6.21
[librubyvm:113339] Finished generating 'librubyvm' in 3m 29s.
Xms3g:
[librubyvm:116953] GC warning: 100.7s spent in 254 GCs during the last stage, taking up 67.26% of the time.
[librubyvm:116953]             Please ensure more than 3.88GB of memory is available for Native Image
[librubyvm:116953]             to reduce GC overhead and improve image build time.

EE:
Xms6g:
18.8s (6.3% of total time) in 349 GCs | Peak RSS: 7.94GB | CPU load: 6.65
[librubyvm:138943] Finished generating 'librubyvm' in 5m 0s.
Xms5g:
45.3s (13.0% of total time) in 483 GCs | Peak RSS: 6.72GB | CPU load: 7.04
[librubyvm:144618] Finished generating 'librubyvm' in 5m 48s.

Author: eregon

mx.truffleruby/mx_truffleruby.py

@@ -313,10 +313,14 @@ def verify_ci(args):
             jar_distributions=['truffleruby:TRUFFLERUBY-LAUNCHER'],
             main_class='org.truffleruby.launcher.RubyLauncher',
             build_args=[
+                '-J-Xmx5g', # Set Xmx to use a reliable amount of memory
                 '-H:+DumpThreadStacksOnSignal',
                 '-H:+DetectUserDirectoriesInImageHeap',
                 '-H:+TruffleCheckBlockListMethods'
             ],
+            build_args_enterprise=[
+                '-J-Xmx6g', # Set Xmx to use a reliable amount of memory
+            ],
             language='ruby',
             option_vars=[
                 'RUBYOPT',