Override virtual function for ARM EH directives.
Link .ARM.exidx with corresponding text section.

The sh_link field in ELF section header of .ARM.exidx
should point to corresponding text section.
Implement ARM unwind opcode assembler.
ARM EHABI workaround on ARM Cortex-A8 processors.

This is a workaround for the ARM processors with
return address stack predictors.  If the subtarget
supports RAS, then the code generator will emit t2B,
and will result in corrupted instruction after linking.
[WORK IN PROGRESS] Add a tool to dump ARM unwind opcodes.