cmd/link/internal/ld, syscall: use libc based msync on darwin for Go ≥ 1.20

cmd/link/internal/ld, syscall: use libc based msync on darwin for Go ≥ 1.20

Direct syscalls should no longer be used on darwin. Instead, directly
call libc's msync when using Go ≥ 1.20 for bootstrap.

For #54265

Change-Id: Ie3f1e6ccd1a06e7f0ddd88cdef5067393a69e8db
Reviewed-on: https://go-review.googlesource.com/c/go/+/430336
Reviewed-by: Ian Lance Taylor <iant@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Auto-Submit: Tobias Klauser <tobias.klauser@gmail.com>