Check for PaX during initialization

[Mikael Vidstedt]

Please review the following change, which adds code to check for the presence of PaX/MPROTECT (https://pax.grsecurity.net/docs/mprotect.txt <https://pax.grsecurity.net/docs/mprotect.txt>) during VM initialization, and prints out a helpful message if PaX is enabled and interfering with the VM. Specifically, the code checks if a writable page can be made executable (mimicking what the JIT would typically do). Since ZERO doesn’t generate code the check is not performed there.

Copy+paste from the actual change:
// Some linux distributions (notably: Alpine Linux) include the
// grsecurity in the kernel by default. Of particular interest from a
// JVM perspective is PaX (https://pax.grsecurity.net/), which adds
// some security features related to page attributes. Specifically,
// the MPROTECT PaX functionality
// (https://pax.grsecurity.net/docs/mprotect.txt) prevents dynamic
// code generation by disallowing a (previously) writable page to be
// marked as executable. This is, of course, exactly what HotSpot does
// for both JIT compiled method, as well as for stubs, adapters, etc.
//
// Instead of crashing "lazily" when trying to make a page executable,
// this code probes for the presence of PaX and reports the failure
// eagerly.

http://cr.openjdk.java.net/~mikael/webrevs/portola/paxcheck/webrev.02/hotspot/webrev/

Cheers,
Mikael