Cygwin: decouple cygheap from Cygwin DLL

Cygwin: decouple cygheap from Cygwin DLL

One reason that ASLR is tricky is the fact that the cygheap
is placed at the end of the DLL and especially that it's expected
to be growable.  To support ASLR, this construct must go.

Define dedicated cygheap memory region and reserve entire region.
Commit 3 Megs, as was the default size of the cygheap before.

Fix linker script accordingly, drop a now useless version check
in get_cygwin_startup_info().