Fix Windows stack allocations.

On Windows we use the function `win32AllocStack` to do stack
allocations in 4k blocks and insert a stack check afterwards
to ensure the allocation returned a valid block.

The problem is this function does something that by C semantics
is pointless. The stack allocated value can never escape the
function, and the stack isn't used so the compiler just optimizes
away the entire function body.

After considering a bunch of other possibilities I think the simplest
fix is to just disable optimizations for the function.

Alternatively inline assembly is an option but the stack check function
doesn't have a very portable name as it relies on e.g. `libgcc`.

Thanks to Sergey Vinokurov for helping diagnose and test.

Test Plan: ./validate

Reviewers: bgamari, erikd, simonmar

Reviewed By: bgamari

Subscribers: rwbarton, thomie, carter

GHC Trac Issues: #14669

Differential Revision: https://phabricator.haskell.org/D4343

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