gas: don't fail due to local register symbols

The diagnostic text as well as its origin are pretty clear: This is about
global symbols. This is further supported by S_IS_LOCAL() returning true
for symbols in reg_section. Add the missing check, adjusting the testcase
that was introduced back at the time (where the sole diagnostic originally
issued was therefore wrong, while other diagnostics were missing, but got
added thanks to work done elsewhere). Further drop the bogus trailing .equ
in another testcase, which were apparently put there to avoid tripping
this or some other undue check (albeit no error surfaced there already
before the change here).

While there also fully eliminate the redundant "sname": There's "name"
already, getting set up a little earlier.