Implement AsmParser for ARM unwind directives.

This commit implements fnstart, fnend, cantunwind, personality,
handlerdata, pad, setfp, save, and vsave directives.
Switch section after ARM unwind opcode is emitted.

We should switch the section to the text section defining
the function.  Otherwise, the continuing function will be
emitted in the .ARM.exidx section, which is incorrect.
Add comments to testcase eh-directive-cantunwind.s.
Add testcases to test combinations of .section, .fnstart, and .fnend.
Add testcase for comdat section with unwind opcode.