Windows: Secp256k1 tests assembly test frozen

[mratsim]

Since #444 the tests on Windows are frozen with Secp256k1 assembly:

This is blocking #445 as it changes the assembly.

The big difference for secp256k1 is that due to using the full 256-bit the final substraction needs to handle an extra underflow condition here:

constantine/constantine/math/arithmetic/assembly/limbs_asm_modular_x86.nim

Lines 55 to 91 in fef6fdc

	proc finalSubMayOverflowImpl*(
	ctx: var Assembler_x86,
	r: Operand or OperandArray,
	a, M, scratch: OperandArray,
	a_in_scratch = false,
	scratchReg: Operand or Register or OperandReuse = rax) =
	## Reduce `a` into `r` modulo `M`
	## To be used when the final substraction can
	## also overflow the limbs (a 2^256 order of magnitude modulus stored in n words of total max size 2^256)
	##
	## r, a, scratch are mutated
	## M is read-only
	## This clobbers RAX
	let N = M.len
	ctx.comment "Final substraction (may carry)"
	# Mask: scratchReg contains 0xFFFF or 0x0000
	ctx.sbb scratchReg, scratchReg
	# Now substract the modulus, and test a < p with the last borrow
	if not a_in_scratch:
	ctx.mov scratch[0], a[0]
	ctx.sub scratch[0], M[0]
	for i in 1 ..< N:
	if not a_in_scratch:
	ctx.mov scratch[i], a[i]
	ctx.sbb scratch[i], M[i]
	# If it overflows here, it means that it was
	# smaller than the modulus and we don't need `scratch`
	ctx.sbb scratchReg, 0
	# If we borrowed it means that we were smaller than
	# the modulus and we don't need "scratch"
	for i in 0 ..< N:
	ctx.cmovnc a[i], scratch[i]
	ctx.mov r[i], a[i]