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

[Nick Clifton]

Hi Thomas,

> 2017-02-03  Thomas preud'homme  <thomas.preudhomme@arm.com>
> 
>     * config/tc-arm.: (md_begin): Set selected_cpu from *mcpu_cpu_opt when
>     CPU_DEFAULT is defined.

Patch approved - please apply.  (Plus apologies for the sloppy thinking that 
insitgated this bug in the first place).

As a matter of interest have you compared the gas/binutils/ld testsuite results
for a binutils compiled with CFLAGS="-DCPU_DEFAULT=ARM_ARCH_V6SM" and without ?
Are there any discrepancies ?  (Post applying your patch that is).

Cheers
  Nick