Add ARM EHABI support to gcc_personality_v0.

Until now the only exception APIs supported by gcc_personality_v0
are DWARF EH and SJLJ. This adds support for ARM EHABI as well.

This is achieved by
a) changing the function signature on ARM EHABI,
b) unwinding the stack before returning _URC_CONTINUE_UNWIND.

See "Exception Handling ABI for the ARM Architecture" for details
(http://infocenter.arm.com/help/topic/com.arm.doc.ihi0038b/IHI0038B_ehabi.pdf).

Patch by Timon Van Overveldt.

llvm-svn: 263010