rts: Add getPhysicalMemorySize
rts: Print correct stack size on stack overflow

This requires that stackOverflow() in RtsUtils.c be passed a reference
to the current TSO. This requires a small change in libraries/base.
rts: Fix typo in comment
rts: Allow for infinite stack size

This is encoded as RtsFlags.GcFlags.maxStkSize == 0.
users_guide: Document zero stack size limit
rts: Set default maximum stack size to 80% of physical memory
users_guide: Update default maximum stack size