unix: add Msync function on Solaris

Add the Msync function on solaris/amd64.

Change-Id: Idde0f9e693f924fef471acb42afb84e83c4cee76
Reviewed-on: https://go-review.googlesource.com/64370
Reviewed-by: Ian Lance Taylor <iant@golang.org>

unix/syscall_solaris.go

@@ -612,6 +612,7 @@ func IoctlGetTermio(fd int, req uint) (*Termio, error) {
 //sys	Mlock(b []byte) (err error)
 //sys	Mlockall(flags int) (err error)
 //sys	Mprotect(b []byte, prot int) (err error)
+//sys	Msync(b []byte, flags int) (err error)
 //sys	Munlock(b []byte) (err error)
 //sys	Munlockall() (err error)
 //sys	Nanosleep(time *Timespec, leftover *Timespec) (err error)