SysTools: Use "process job" when spawning processes on Windows

GHC should make calls using process jobs when calling out to GCC and LD.
The reason is these use the exec () family of posix functions.  Window's
process model doesn't allow replacement of processes so this is emulated
by creating a new process and immediately exiting the old one.  Because
of this when using normal Windows wait functions you would return even
without the child process having finished.  In this case if you are
depending on data from the child you will enter a race condition.

The usual fix for this is to use process jobs and wait for the
termination of all children that have ever been spawn by the process you
called. But also waiting for the freeing of all resources.

(cherry picked from commit 584eee71)