Support repeated ARM unwind directives.

This commit enhanced the ARM unwind opcode assembler in
several ways:

* Generate correct opcode for multiple .save and .vsave
  directives.

* Fix the calculation of the $sp offset for .setfp and
  .pad directives.

Also, this CL changed several test cases:

* The eh-directive-pad.s and eh-directive-setfp.s are
  changed due to the different order of opcodes.  This
  change do not change the behavior of the generated
  file and it is same as the one generated by gas.

* The ehabi-mc.ll and ehabi-mc-section.s are changed
  because an incorrect $sp offset was placed in the
  test cases.  The new value is same as the one calculated
  by gas now.