Add simple implementation _Unwind_GetIP() for clang compiles.

Clang (prior to 3.4) does not actually provide a declaration (or definition)
of _Unwind_GetIP() for ARM. We can work around this by writing our own
basic implementation using the available primitive operations.

Change-Id: If6c66846952d8545849ad32d2b55daa4599cfe2c