RFR: 8295344: Harden runtime/StackGuardPages/TestStackGuardPages.java

[Johan Sjölen]

There are multiple ways which the stack guard pages tests can be hardened:
- Rewritten the Java part of the test using ProcessTools.executeProcess(ProcessBuilder pb) which will make stdout/stderr be consistently logged and debugging much easier.
- Split up initial/other thread into 2 tests
- Split up Java thread tests and native thread tests into two separate files
- Check more error conditions in the C code
- Instead of recursing and creating stack frames, just `alloca()` and poke until we hit a `SIGSEGV`.

The goal is that this will help us out with  [JDK-8293452](https://bugs.openjdk.org/browse/JDK-8293452).

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Commit messages:
 - Refer to method instead of hard coded printAlive in exeinvoke
 - Separate into two tests
 - Rename, document variables
 - Add _y to peek at allocated space
 - Allocate 128 bytes
 - Only init thread when absolutely necessary
 - Check explicitly
 - Split up test

Changes: https://git.openjdk.org/jdk/pull/12386/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=12386&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8295344
  Stats: 151 lines in 3 files changed: 96 ins; 29 del; 26 mod
  Patch: https://git.openjdk.org/jdk/pull/12386.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/12386/head:pull/12386

PR: https://git.openjdk.org/jdk/pull/12386