gas: arm: handle multiple .directives on a single line (PR29519)

gas: arm: handle multiple .directives on a single line (PR29519)

There's been a long-standing bug in the arm backend where
target-specific directives did not correctly handle lines with
multiple statements.  This patch fixes the issue for all the cases
I've been able to find.

It does result in a slight change in behaviour when errors are
encountered: where, previously,

  .cpu arm6 bar

would result in the error "junk at end of line, first unrecognized
character is `b'", we now get "unknown cpu `arm6 bar'", which I think
is slightly more helpful anyway.  Similar errors are generated for
other directives.