Allow the optional limitation on dmb instructions as is already the case
with dsb instructions.


git-svn-id: svn+ssh://svn.freebsd.org/base/head@272472 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f
Add all the dmb/dsb optional limitations, including the alternative values.
These are needed for some code llvm generates when targeting ARMv7.


git-svn-id: svn+ssh://svn.freebsd.org/base/head@272473 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f