cmd/go/internal/mmap: use syscall.Mmap on solaris

Now that syscall.Mmap is defined on solaris (see CL 413374), use it in
mmapFile like on other Unix ports.

For #52875

Change-Id: Ic5c5a84da8613f0c6dc947a52b7fcca50af43d79
Reviewed-on: https://go-review.googlesource.com/c/go/+/413375
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Reviewed-by: Bryan Mills <bcmills@google.com>

src/cmd/go/internal/mmap/mmap_plan9.go

@@ -2,8 +2,6 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-//go:build plan9 || solaris
-
 package mmap
 
 import (

src/cmd/go/internal/mmap/mmap_unix.go

@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-//go:build unix && !solaris
+//go:build unix
 
 package mmap