cmd, syscall: use syscall.Mmap on solaris for Go ≥ 1.20

cmd, syscall: use syscall.Mmap on solaris for Go ≥ 1.20

CL 413374 added syscall.Mmap on solaris. Use it in cmd/compile and
cmd/link if the bootstrap toolchain is Go ≥ 1.20.

For #52875
For #54265

Change-Id: I9a0534bf97926eecf0c6f1f9218e855344ba158f
Reviewed-on: https://go-review.googlesource.com/c/go/+/430496
Reviewed-by: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Auto-Submit: Tobias Klauser <tobias.klauser@gmail.com>
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>