Set SGen's REDZONE_SIZE=0 on Win64 since the ABI defines no red zone and the

Set SGen's REDZONE_SIZE=0 on Win64 since the ABI defines no red zone and the
ABI states that "All memory beyond the current address of RSP is considered
volatile".

On Windows the stack grows dynamically as the thread needs more stack space.
Windows places a guard page before the page at the top of stack and when this
guard page is accessed by the thread Windows will add a new page to the stack
and resume the thread.

Before this fix SGen on Win64 subtracted 128 from the stack pointer of a
suspended thread and started the stack scan from this address. In the case
when the stack pointer would be less than 128 from the start of the memory
page SGen would attempt to read from the previous page. If this happens to be
the guard page a STATUS_GUARD_PAGE_VIOLATION exception is raised and the
process crashes. The sgen-weakref-stress.exe triggered this bug on almost
every invocation.