Arm: correct macro use in gas testsuite

The way the inner macro invocations are written doesn't quite work as
expected (and would actually break subsequently): Due to overly
aggressive removal of whitespace by the scrubber, the incoming \sym and
\offset arguments actually get concatenated; an empty 3rd argument is
being passed to ldrtest2. That just so happened to work as intended; any
use of \offset alone would have exposed the problem. Quote the 3rd
argument, thus retaining enough whitespace to be independent of scrubber
internals.