runtime: fix stack alignment for Windows amd64 lib entry

Windows amd64 calling convention requires 16-bytes aligned
stack pointer. Before this patch, the real frame size is
0x48 (frame size) + 0x10 (frame pointer & return address),
which does not satisfy the alignment requirement.

_cgo_sys_thread_create eventually calls NtCreateThread,
which receives a pointer to a ThreadContext structure
allocated from (mis-aligned) stack, and may fail with
STATUS_DATATYPE_MISALIGNMENT on some implementations.

BP is saved/restored by prolog/epilog.
AX, CX, DX are volatile, no need to save and restore.

Fixes #41075

Change-Id: I01c0a22b4bf3b4cfdebf4df587445aa46c667973
GitHub-Last-Rev: 15d2bd740e3e61c9753b3e1b574fdb5da538459c
GitHub-Pull-Request: golang/go#44524
Reviewed-on: https://go-review.googlesource.com/c/go/+/295329


Trust: Alex Brainman <alex.brainman@gmail.com>
Trust: Jason A. Donenfeld <Jason@zx2c4.com>
Run-TryBot: Alex Brainman <alex.brainman@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Jason A. Donenfeld <Jason@zx2c4.com>
Reviewed-by: Christopher Nelson <nadiasvertex@gmail.com>