Always use GC_set_stackbottom on Windows

[HertzDevil]

When the Boehm GC pushes all stacks during a collection cycle, with POSIX Threads it uses GC_stackbottom as the bottom of the main thread's stack, but on Windows, it always uses an internal per-thread handle's stack_base member. This member is updated by GC_set_stackbottom. This means it is insufficient to assign directly to LibGC.$stackbottom, because then the GC would incorrectly collect memory reachable only from the main fiber if a different fiber is running while a collection cycle starts.

This fix gets rid of the occasional Thread stack pointer ... out of range, pushing everything warnings, and should also make the select stress test pass.