RFR: 8236569: -Xss not multiple of 4K does not work for the main thread on macOS

[Henry Jen]

…d on macOS

This patch simply round up the specified stack size to multiple of the system page size. 

Test is trivial, simply run java with -Xss option against following code. On MacOS, before the fix, running with `-Xss159k` and `-Xss160k` would get `7183` and `649` respectively. After fix, both would output `649`, while `-Xss161k` would be same as `-Xss164k` and see 691 as the output.

```code:java
public class StackLeak {
    public int depth = 0;
    public void stackLeak() {
        depth++;
        stackLeak();
    }

    public static void main(String[] args) {
        var test = new StackLeak();
        try {
            test.stackLeak();
        } catch (Throwable e) {
            System.out.println(test.depth);
        }
    }
}

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

Commit messages:
 - JDK-8236569: -Xss not multiple of 4K does not work for the main thread on macOS

Changes: https://git.openjdk.java.net/jdk/pull/4256/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=4256&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8236569
  Stats: 23 lines in 2 files changed: 21 ins; 0 del; 2 mod
  Patch: https://git.openjdk.java.net/jdk/pull/4256.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/4256/head:pull/4256

PR: https://git.openjdk.java.net/jdk/pull/4256