RFR: Add a limition to allocate object heap in aarch64 with tsan enabled.

[Jie He]

Vm will calculate max heap size according to the physical ram if user
doesn't specify the size(-Xmx). However, with tsan 48-bits memory
mapping of aarch64, it's hard to allocate the large continuous vm
space. Heap allocation often fail in this case, thus, add a limitation
at the end of the heap size calculation to prevent to exceed the
avaiable size. Currently, only support 48-bits address space.

Can reproduce the behavior by launching "java --version" in a machine
with huge physical memory.

-------------

Commit messages:
 - Add a limition to allocate object heap in aarch64 with tsan enabled.

Changes: https://git.openjdk.java.net/tsan/pull/6/files
 Webrev: https://webrevs.openjdk.java.net/tsan/6/webrev.00
  Stats: 17 lines in 1 file changed: 17 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/tsan/pull/6.diff
  Fetch: git fetch https://git.openjdk.java.net/tsan pull/6/head:pull/6

PR: https://git.openjdk.java.net/tsan/pull/6