Drop the thumb bit from the IP when finding the EHTable.

In UnwindCursor::setInfoBasedOnIPRegister, we subtract one
from the pc to keep the pc from going beyond the end of the
method.

On arm if in thumb mode, subtracting one only removes the
thumb and leaves us in the same position. So we clear the thumb
bit before subtracting to ensure we move the instruction pointer
to the previous instruction.

This fixes the last failing libc++abi test.

Change-Id: I000fb3a698db77caa4950532c20aab4b0fa486f8
Merge pull request #77 from awong-dev/thumb

Drop the thumb bit from the IP when finding the EHTable.