arm: rework fix for PR gas/15273

PR gas/15273

The apparent intent of the original code added in
https://sourceware.org/pipermail/binutils/2012-August/078044.html was
to emit an error message if the instruction was obsolete and only to
emit a deprecation warning if an error hadn't already been emitted.
However, when the insn has not yet been obsoleted, the code would
generate a warning with a (null) message body if the selected CPU was
'any'.

The previous fix for this bug was to remove the support for the
obsoletion message entirely, which was probably the wrong approach.  A
better fix is to only call check_obsolete if obs_msg is non-null; we
can use this as a proxy for the instruction not being obsolete on any
architecture.

While we are here, fix an incorrect capitalization in the deprecation
message ('This' to 'this').