cmd/compile: add prefetch intrinsic support

This CL provide new intrinsics to emit prefetch instructions for AMD64
and ARM64 platforms:
Prefetch - prefetches data from memory address to cache;
PrefetchStreamed - prefetches data from memory address, with a hint
that this data is being streamed.

This patch also provides prefetch calls pointed by RSC inside scanobject
and greyobject of GC mark logic.

Performance results provided by Michael:
https://perf.golang.org/search?q=upload:20210901.9

Benchmark parameters:
tree2 -heapsize=1000000000 -cpus=8
tree -n=18
parser
peano

Benchmarks AMD64 (Xeon - Cascade Lake):
name        old time/op  new time/op  delta
Tree2-8     36.1ms ± 6%  33.4ms ± 5%  -7.65%  (p=0.000 n=9+9)
Tree-8       326ms ± 1%   324ms ± 1%  -0.44%  (p=0.006 n=9+10)
Parser-8     2.75s ± 1%   2.71s ± 1%  -1.47%  (p=0.008 n=5+5)
Peano-8     63.1ms ± 1%  63.0ms ± 1%    ~     (p=0.730 n=9+9)
[Geo mean]   213ms        207ms       -2.45%

Benchmarks ARM64 (Kunpeng 920):
name        old time/op  new time/op  delta
Tree2-8     50.3ms ± 8%  44.1ms ± 5%  -12.24%  (p=0.000 n=10+9)
Tree-8       494ms ± 1%   493ms ± 1%     ~     (p=0.684 n=10+10)
Parser-8     3.99s ± 1%   3.93s ± 1%   -1.37%  (p=0.016 n=5+5)
Peano-8     84.4ms ± 0%  84.1ms ± 1%     ~     (p=0.068 n=8+10)
[Geo mean]   302ms        291ms        -3.67%

Change-Id: I43e10bc2f9512dc49d7631dd8843a79036fa43d0
Reviewed-on: https://go-review.googlesource.com/c/go/+/328289


Reviewed-by: Austin Clements <austin@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Go Bot <gobot@golang.org>