unix: add unsafe mmap, munmap, mremap

Fixes golang/go#56123

Change-Id: I63a7a6fb3a5b1bb556ac19d76a1e0b04a03ebcfa
GitHub-Last-Rev: 39dbc8e308807bd07863b6f5a49bcdcf506bc0e7
GitHub-Pull-Request: golang/sys#197
Reviewed-on: https://go-review.googlesource.com/c/sys/+/592415
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>

unix/syscall_unix.go

@@ -154,6 +154,15 @@ func Munmap(b []byte) (err error) {
 	return mapper.Munmap(b)
 }
 
+func MmapPtr(fd int, offset int64, addr unsafe.Pointer, length uintptr, prot int, flags int) (ret unsafe.Pointer, err error) {
+	xaddr, err := mapper.mmap(uintptr(addr), length, prot, flags, fd, offset)
+	return unsafe.Pointer(xaddr), err
+}
+
+func MunmapPtr(addr unsafe.Pointer, length uintptr) (err error) {
+	return mapper.munmap(uintptr(addr), length)
+}
+
 func Read(fd int, p []byte) (n int, err error) {
 	n, err = read(fd, p)
 	if raceenabled {