cmd/compile: set unalignedOK to make memcombine work properly on loong64

cmd/compile: set unalignedOK to make memcombine work properly on loong64

goos: linux
goarch: loong64
pkg: unicode/utf8
cpu: Loongson-3A6000-HV @ 2500.00MHz
                            │     old     │                 new                 │
                            │   sec/op    │   sec/op     vs base                │
ValidTenASCIIChars            7.604n ± 0%   6.805n ± 0%  -10.51% (p=0.000 n=10)
Valid100KASCIIChars           37.41µ ± 0%   16.58µ ± 0%  -55.67% (p=0.000 n=10)
ValidTenJapaneseChars         60.84n ± 0%   58.62n ± 0%   -3.64% (p=0.000 n=10)
ValidLongMostlyASCII          113.5µ ± 0%   113.5µ ± 0%        ~ (p=0.303 n=10)
ValidLongJapanese             204.6µ ± 0%   206.8µ ± 0%   +1.07% (p=0.000 n=10)
ValidStringTenASCIIChars      7.604n ± 0%   6.803n ± 0%  -10.53% (p=0.000 n=10)
ValidString100KASCIIChars     38.05µ ± 0%   17.14µ ± 0%  -54.97% (p=0.000 n=10)
ValidStringTenJapaneseChars   60.58n ± 0%   59.48n ± 0%   -1.82% (p=0.000 n=10)
ValidStringLongMostlyASCII    113.5µ ± 0%   113.4µ ± 0%   -0.10% (p=0.000 n=10)
ValidStringLongJapanese       205.9µ ± 0%   207.3µ ± 0%   +0.67% (p=0.000 n=10)
geomean                       3.324µ        2.756µ       -17.08%

Change-Id: Id43b6e2e41907bd4b92f421dacde31f048db47d6
Reviewed-on: https://go-review.googlesource.com/c/go/+/662495
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Keith Randall <khr@golang.org>
Auto-Submit: Keith Randall <khr@golang.org>
Reviewed-by: abner chenc <chenguoqi@loongson.cn>
Reviewed-by: Keith Randall <khr@google.com>