ARM IAS: support .personalityindex

The .personalityindex directive is equivalent to the .personality directive with
the ARM EABI personality with the specific index (0, 1, 2).  Both of these
directives indicate personality routines, so enhance the personality directive
handling to take into account personalityindex.

Bonus fix: flush the UnwindContext at the beginning of a new function.

Thanks to Logan Chien for additional tests!
ARM IAS: add support for .unwind_raw directive

This implements the unwind_raw directive for the ARM IAS.  The unwind_raw
directive takes the form of a stack offset value followed by one or more bytes
representing the opcodes to be emitted.  The opcode emitted will interpreted as
if it were assembled by the opcode assembler via the standard unwinding
directives.

Thanks to Logan Chien for an extra test!
tools: support decoding ARM EHABI opcodes in readobj

Add support to llvm-readobj to decode the actual opcodes.  The ARM EHABI opcodes
are a variable length instruction set that describe the operations required for
properly unwinding stack frames.

The primary motivation for this change is to ease the creation of tests for the
ARM EHABI object emission as well as the unwinding directive handling in the ARM
IAS.

Thanks to Logan Chien for an extra test case!