jspawnhelper should be built on all unix platforms

We should match the building of jspawnhelper in make/modules/java.base/Launcher.gmk with the usage for all unix platforms in src/java.base/unix/classes/java/lang/ProcessImpl.java.

Granted, the list of OSes in the makefile amounts to the current list of all Unix OSes in the JDK, but there is no need to have this logical disparity.