math: remove asm version of sincos everywhere, except 386

math: remove asm version of sincos everywhere, except 386

We have dedicated asm implementation of sincos only on 386 and amd64,
on everything else we are just jumping to generic version.
However amd64 version is actually slower than generic one:

Sincos-6               34.4ns ± 0%   24.8ns ± 0%  -27.79%  (p=0.000 n=8+10)

So remove all sincos*.s and keep only generic and 386.

Updates #19819

Change-Id: I7eefab35743729578264f52f6d23ee2c227c92a5
Reviewed-on: https://go-review.googlesource.com/41200
Run-TryBot: Ilya Tocar <ilya.tocar@intel.com>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>