fat_init violates host ABI on Win64

[Niels Möller]

I wonder if one could use some m4 macrology to reduce clutter. Maybe
one macro for the pattern

IFSTD(`	sub	$8, %rsp	')
IFDOS(`	sub	$40, %rsp	')
	ASSERT(nz, `test $15, %rsp')
	CALL(	mpn_add_n)
IFSTD(`	add	$8, %rsp	')
IFDOS(`	add	$40, %rsp	')

used when it's known that rsp = 8 (mod 16), and another one when it's
known that rsp = 0 (mod 16).

There seems to be a few uses of CALL that want to do this differently,
though, e.g., gcd_1.asm, maybe it would also help to move (and rename)
its conditional definition of STACK_ALLOC.

/Niels

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