cmd/compile: allow equal functions to perform unaligned load on loong64

cmd/compile: allow equal functions to perform unaligned load on loong64

The Go compiler always generates Loong64 binaries, which can execute any
processor of LA364, LA464, LA664 or higher core. And these processors
support unaligned memory access [1].

goos: linux
goarch: loong64
pkg: strings
cpu: Loongson-3A6000 @ 2500.00MHz
              |   old.txt    |               new.txt                |
              |    sec/op    |    sec/op     vs base                |
StringPrefix3   4.0040n ± 0%   0.4003n ± 0%  -90.00% (p=0.000 n=10)
StringPrefix5   4.0040n ± 0%   0.4003n ± 0%  -90.00% (p=0.000 n=10)
StringPrefix6   3.6030n ± 0%   0.4002n ± 0%  -88.89% (p=0.000 n=10)
StringPrefix7   4.0040n ± 0%   0.4002n ± 0%  -90.00% (p=0.000 n=10)
geomean          3.900n        0.4003n       -89.74%

goos: linux
goarch: loong64
pkg: strings
cpu: Loongson-3A5000-HV @ 2500.00MHz
              |   old.txt    │               new.txt                |
              |    sec/op    │    sec/op     vs base                |
StringPrefix3   5.6160n ± 0%   0.4011n ± 0%  -92.86% (p=0.000 n=10)
StringPrefix5   5.6180n ± 0%   0.4011n ± 0%  -92.86% (p=0.000 n=10)
StringPrefix6   5.2170n ± 0%   0.4011n ± 0%  -92.31% (p=0.000 n=10)
StringPrefix7   5.6170n ± 0%   0.4009n ± 0%  -92.86% (p=0.000 n=10)
geomean          5.514n        0.4010n       -92.73%

goos: linux
goarch: loong64
pkg: strings
cpu: Loongson-3B6000M @ 2400.00MHz
              |   old.txt    │               new.txt                |
              |    sec/op    │    sec/op     vs base                |
StringPrefix3   5.0060n ± 0%   0.4223n ± 1%  -91.56% (p=0.000 n=10)
StringPrefix5   4.5890n ± 0%   0.4214n ± 0%  -90.82% (p=0.000 n=10)
StringPrefix6   4.5890n ± 0%   0.4190n ± 1%  -90.87% (p=0.000 n=10)
StringPrefix7   4.5890n ± 0%   0.4226n ± 1%  -90.79% (p=0.000 n=10)
geomean          4.690n        0.4213n       -91.02%

[1]: https://go.dev/wiki/MinimumRequirements#loong64

Change-Id: I1870080e0122a7d136685e3045699d0cf1e4194d
Reviewed-on: https://go-review.googlesource.com/c/go/+/742260
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Meidan Li <limeidan@loongson.cn>
Reviewed-by: Michael Pratt <mpratt@google.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Auto-Submit: Michael Pratt <mpratt@google.com>