Currently, on Linux, Runtime.exec() uses fork() or vfork(), with vfork() being the default. This is controlled by -Djdk.lang.Process.launchMechanism, with allowed values of "FORK" and "VFORK" on linux.
Enable the use of posix_spawn() on Linux as it is used already on Solaris, BSD and AIX. But do not change the default.
Purpose of this patch is to be able to test posix_spawn() on Linux, with the expressed wish to change the default to posix_spawn() later.
See also discussion thread: http://mail.openjdk.java.net/pipermail/core-libs-dev/2018-October/056122.html