Arm: avoid unhelpful uses of .macro in testsuite

Arm: avoid unhelpful uses of .macro in testsuite

Macros with just a single use site are a little pointless to have, and
even in further cases .irp is more suitable for the purpose. Expand such
inline, avoiding the need to touch the testcases when diagnostics are
changed for code resulting from macro expansion.

While there also make what was "iter_mla" in sp-usage-thumb2-relax cover
smlatt as well, rather than testing smlabt twice.