RFR: 8252500: ZGC on aarch64: Unable to allocate heap for certain Linux kernel configurations

[Christoph Göttschkes]

The patch introduces a new function to probe for the highest valid bit in the virtual address space for userspace
programs on Linux.

I guarded the whole implementation to only probe on Linux, other platforms will remain unaffected. Possibly, it would
be nicer to move the probing code into an OS+ARCH specific source file. But since this is only a single function, I
thought it would be better to put it right next to the caller and guard it with an #ifdef LINUX.

The probing mechanism uses a combination of msync + mmap, to first check if the address is valid using msync (if msync
succeeds, the address was valid). If msync fails, mmap is used to check if msync failed because the memory wasn't
mapped, or if it failed because the address is invalid. Due to some undefined behavior (documented in the msync man
page), I also use a single mmap at the end, if the msync approach failed before.

I tested msync with different combinations of mappings, and also with sbrk, and it always succeeded, or failed with
ENOMEM. I never got back any other error code.

The specified minimum value has been chosen "randomly". The JVM terminates (unable to allocate heap), if this minimum
value is smaller than the requested Java Heap size, so it might be better to make the minimum dependent on the
MaxHeapSize and not a compile time constant? I didn't want to make the minimum too big, since for aarch64 on Linux, the
documented minimum would be 38 (see [1]).

I avoided MAP_FIXED_NOREPLACE, because according to the man page, it has been added in Linux 4.17. There are still
plenty of stable kernel versions around which will not have that feature, which means we need to implement a workaround
for it. Some of my test devices also have a kernel version lower than that.

I executed the HotSpot tier1 JTreg tests on two different aarch64 devices. One with 4KB pages and 3 page levels and the
other with 4KB pages and 4 page levels. Tests passed on both devices.

[1] https://www.kernel.org/doc/Documentation/arm64/memory.txt

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Commit messages:
 - 8252500: ZGC on aarch64: Unable to allocate heap for certain Linux kernel configurations

Changes: https://git.openjdk.java.net/jdk/pull/40/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=40&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8252500
  Stats: 62 lines in 1 file changed: 60 ins; 0 del; 2 mod
  Patch: https://git.openjdk.java.net/jdk/pull/40.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/40/head:pull/40

PR: https://git.openjdk.java.net/jdk/pull/40