RFR: Disable ShHeapUncommit on too-large large-pages

[Roman Kennke]

On my aarch64-test-box I get large pages of 512MB. When running the
HeapUncommit test with just -Xmx1g and -XX:+UseLargePages we attempt to
have at least large-page-size-sized regions, which would result in just
2 regions, which is not possible (and rightly trips an assert).

The reason why we need to do this is for heap-region-uncommit: if we had
regions smaller than page size, we would accidently uncommit adjacent
regions too.

After some discussions with Aleksey off-list, I propose to disable heap
region uncommit if large-page-size is too large:

http://cr.openjdk.java.net/~rkennke/disable-uncommit/webrev.00/

Testing: tier3_gc_shenandoah/aarch64 (except 1 remaining test that fails
for other reasons)

Good?

Roman