[ARM] Update selected_cpu based on info got during parsing

[ARM] Update selected_cpu based on info got during parsing

  gas/
    * config/tc-arm.c (aeabi_set_public_attributes): Update intended_arch based
    on the info we got during parsing.
    (arm_handle_align): Make sure the p2align expanding logic under thumb
    unchanged.

  gas/testsuite/
    * gas/arm/blx-bl-convert.d: New testcase.
    * gas/arm/blx-bl-convert.l: Warning expectation.
    * gas/arm/blx-bl-convert.s: Source file.