cmd/compile,math: remove all sqrt assembly code

cmd/compile,math: remove all sqrt assembly code

This CL make math.sqrt an intrinsic function, math.Sqrt is not affected
since compiler can inline it. With this change, we can remove all assembly
code for math.Sqrt that aims to speed up indirect call. The go compiler can
generate same or faster code (with regabi) for indirect call.

Benchmark on amd64:
name                 old time/op  new time/op  delta
SqrtIndirect         2.60ns ± 3%  1.03ns ± 4%  -60.24%  (p=0.000 n=10+10)
SqrtLatency          3.40ns ± 1%  3.32ns ± 1%   -2.26%  (p=0.000 n=10+8)
SqrtIndirectLatency  6.09ns ± 0%  3.31ns ± 0%  -45.67%  (p=0.000 n=10+10)
SqrtGoLatency        36.1ns ± 6%  34.6ns ± 1%     ~     (p=0.101 n=10+10)
SqrtPrime            2.53µs ± 2%  2.55µs ± 6%     ~     (p=0.398 n=9+9)

Change-Id: If4be0f242c1d9d4feca7d269fc9cd6e6066f163d
Reviewed-on: https://go-review.googlesource.com/c/go/+/421074
Run-TryBot: Wayne Zuo <wdvxdr@golangcn.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Keith Randall <khr@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>