as: add deprecation notice to the man page

In the future FreeBSD will ship without GNU binutils 2.17.50. Add a
note advising users who require GNU as to install the binutils port
or package.

Note that on armv7, arm64, amd64, i386 we currently ship only two
binutils tools (as and objdump). A deprecation notice was added to
objdump's man page some time ago.

PR: 233611
Discussed with: jhb
MFC after: 1 week
Relnotes: Yes
Sponsored by: The FreeBSD Foundation