Fix ARM unwind opcode assembler in several cases.

Changes to ARM unwind opcode assembler:

* Fix multiple .save or .vsave directives.  Besides, the
  order is preserved now.

* For the directives which will generate multiple opcodes,
  such as ".save {r0-r11}", the order of the unwind opcode
  is fixed now, i.e. the registers with less encoding value
  are popped first.

* Fix the $sp offset calculation.  Now, we can use the
  .setfp, .pad, .save, and .vsave directives at any order.

Changes to test cases:

* Add test case to check the order of multiple opcodes
  for the .save directive.

* Fix the incorrect $sp offset in the test case.  The
  stack pointer offset specified in the test case was
  incorrect.  (ehabi-mc-section.ll and ehabi-mc.ll)

* The opcode to restore $sp are slightly reordered.  The
  behavior are not changed, and the new output is same
  as the output of GNU as.  (eh-directive-pad.s and
  eh-directive-setfp.s)
Refine the ARM EHABI test cases.

Since we have ARM unwind directive parser and assembler, we
can test the correctness in two stages:

1. From LLVM assembly (.ll) to ARM assembly (.s)
2. From ARM assembly (.s) to ELF object file (.o)

We already have several .s to .o test cases.  This change adds
some .ll to .s test cases and removes the redundant .ll to .o
test cases.

New test cases to test .ll to .s:

- ehabi.ll: Check the correctness of the generated unwind directives.
- section-func.ll: Check the section directives.

Removed test cases:

- ehabi-mc-cantunwind.ll
  (covered by ehabi-cantunwind.ll, and eh-directive-cantunwind.s)
- ehabi-mc-compact-pr0.ll
  (covered by ehabi.ll, eh-compact-pr0.s, eh-directive-save.s, and
   eh-directive-setfp.s)
- ehabi-mc-compact-pr1.ll
  (covered by ehabi.ll, eh-compact-pr1.s, eh-directive-save.s, and
   eh-directive-setfp.s)
- ehabi-mc.ll
  (covered by ehabi.ll, and eh-directive-integrated-test.s)
- ehabi-mc-section-group.ll
  (covered by section-func.ll, and eh-directive-section-comdat.s)
- ehabi-mc-section.ll
  (covered by section-func.ll, and eh-directive-section.s)
- ehabi-mc-sh_link.ll
  (covered by eh-directive-text-section.s, and eh-directive-section.s)