syscall: add Mmap and Munmap on solaris

syscall: add Mmap and Munmap on solaris

They exist on all other Unix ports, define them on GOOS=solaris as well.

Fixes #52875

Change-Id: I7285156b3b48ce12fbcc6d1d88865540a5c51a21
Reviewed-on: https://go-review.googlesource.com/c/go/+/413374
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Auto-Submit: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Than McIntosh <thanm@google.com>
Reviewed-by: Bryan Mills <bcmills@google.com>