8201495: [Zero] Reduce limits of max heap size for boot JDK on s390

[Severin Gehwolf]

Hi,

We (Red Hat) have been building Zero on s390 for a while now. In order
to do so we needed to have this patch to reduce the maximum heap size
setting for big workloads. Otherwise we see this during (JDK 9) builds:

++ /usr/bin/tee /builddir/build/BUILD/java-9-openjdk-9.0.4.12-5.openjdk9.el7.s390/openjdk/build/jdk/modules/java.base/_the.java.base_batch.log
++ /usr/bin/tee /builddir/build/BUILD/java-9-openjdk-9.0.4.12-5.openjdk9.el7.s390/openjdk/build/jdk/modules/java.base/_the.java.base_batch.log
Error occurred during initialization of VM
Could not reserve enough space for 1048576KB object heap

NOTE: JDK 9 has the same build logic than JDK 11 in terms of big
workloads' JVM switches.

Bug: https://bugs.openjdk.java.net/browse/JDK-8201495
webrev: http://cr.openjdk.java.net/~sgehwolf/webrevs/JDK-8201495/webrev.01/

Testing: Run configure on s390 and inspected the big workloads settings:

Before:
checking flags for bootcycle boot jdk java command for big workloads... -Xms64M -Xmx998M -XX:ThreadStackSize=768

After:
checking flags for bootcycle boot jdk java command for big workloads... -Xms256M -Xmx768M -XX:ThreadStackSize=768

This should be fairly low risk, since the check is guarded by s390
archicture checks. Other architectures should be unaffected.

Thanks,
Severin