[GAS/ARM] Fix selected_cpu with default CPU and -mcpu

[GAS/ARM] Fix selected_cpu with default CPU and -mcpu

When GAS is compiled with DEFAULT_CPU set and then run with a -mcpu or
-march option, selected_cpu will be set to the default CPU. This means
the -mcpu is ignored which is surprising behavior. This commit instead
sets selected_cpu from the value passed to -mcpu/-march.

2017-03-24  Thomas preud'homme  <thomas.preudhomme@arm.com>

gas/
	* config/tc-arm.: (md_begin): Set selected_cpu from *mcpu_cpu_opt when
	CPU_DEFAULT is defined.