[ARM] Simplify Tag_DSP_extension selection logic

[ARM] Simplify Tag_DSP_extension selection logic

=== Context ===

This patch is part of a patch series to add support for ARMv8-R
architecture. Its purpose is to simplify the logic to decide whether to
set Tag_DSP_extension.

=== Motivation ===

To decide whether to set Tag_DSP_extension, the current code was
checking whether the flags had DSP instruction but the architecture
selected for Tag_CPU_arch did not have any. This was necessary because
extension feature bit were not available separately. This is no longer
necessary and can be simplified.

=== Patch description ===

The patch change the logic to set Tag_DSP_extension to check whether any
DSP feature bit is set in the extension feature bit, as per the
definition of that build attribute. The patch also removes all
definitions of arm_arch which is now unneeded.

2017-06-21  Thomas Preud'homme  <thomas.preudhomme@arm.com>

gas/
	* config/tc-arm.c (aeabi_set_public_attributes): Test *mcpu_ext_opt to
	decide whether to set Tag_DSP_extension build attribute value.  Remove
	now useless arm_arch variable.