runtime: ensure stack is aligned in _rt0_amd64_windows_lib

[ferrmin]

The Windows DLL loader may call a DLL entry point, in our case _rt0_amd64_windows_lib, with a stack that is
not 16-byte aligned. In theory, it shouldn't, but under some circumstances, it does (see below how to reproduce it).

Having an unaligned stack can, and probably will, cause problems down the line, for example if a movaps instruction tries to store a value in an unaligned address it throws an Access Violation exception (code 0xc0000005).

I managed to consistently reproduce this issue by loading a Go DLL into a C program that has the Page Heap Verification diagnostic enabled [1].

Updates golang#54187 (and potentially fixes)

[1] https://learn.microsoft.com/en-us/windows-hardware/drivers/debugger/example-12---using-page-heap-verification-to-find-a-bug

Change-Id: Id0fea7f407e024c9b8cdce10ce4802d7535e7542
Reviewed-on: https://go-review.googlesource.com/c/go/+/510755
TryBot-Result: Gopher Robot [email protected]
Reviewed-by: Cherry Mui [email protected]
Reviewed-by: Than McIntosh [email protected]
Run-TryBot: Quim Muntal [email protected]

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