modula-2: Fix stack size request in initPreemptive [PR108405]

modula-2: Fix stack size request in initPreemptive [PR108405]

As noted in the PR, the problem is that we make a request for additional
stack that violates the constraints on some systems.

This patch chooses a value that is divisible by common OS page sizes.

TODO: the user value should be checked and then an exception thrown if it
is not suitable.

Signed-off-by: Iain Sandoe <iain@sandoe.co.uk>
	PR modula2/108405

gcc/m2/ChangeLog:

	* gm2-libs-iso/Preemptive.mod (initPreemptive): Use a value for
	extra space that is divisible by common OS pagesizes.