Teach as(1) to handle the arm .arch_extension pseudo-op, which accepts

Teach as(1) to handle the arm .arch_extension pseudo-op, which accepts
the same values as the -march= command line option.  Add support for the
"sec" extension (security extensions).

We've been getting away without support for the sec extension because
it's bogusly enabled even on arches where its presence is optional.  This
support for .arch_extension is being added mainly so that we can use the
right directives in our source code, and that helps folks using external
toolchains (and will help us when we finally update our toolchain).