Replacing "redc"

[Torbjörn Granlund]

I pushed new files for 2-adic division, using the agreed-upon semantics
and interface.  I expanded _q and _qr to a pure _r variant, in order to
lower the register pressure for asm variants (no need for a qp
parameter).

The only asm file so far is for AMD Zen.  This is a more thorough
implementation than our old redc_1.asm code.  Thew new code has special
loops for operands up to 8 limbs, and also does software pipelining of
the quotient computation.  (The final quotient limb computation will be
wasted, but that's no real harm.)

The new code runs just a tad bit slower than plain mul_basecase.

We should use this in lieu of redc_1 in mpn/generic/powm.c and
mpn/generic/sec_powm.c.  It is a non-trivial thing, as redc_1 and
sbpi1_bdiv_r leaves the remainder in different places; redc_1 puts it in
place of the low input dividend limbs while sbpi1_bdiv_r puts it in
toward the upper end of the same operand.

The end goal is to get rid pf the redc_* interfaces completely.

-- 
Torbjörn
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