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>