[libunwind] [SEH] Implement parsing of ARM pdata/xdata (#137950)

This is generally very similar to the aarch64 case.

Contrary to aarch64, the public headers don't contain any definition of
a struct for interpreting this data, so we provide our own.

GitOrigin-RevId: fc83aaea06eb06f73ac53491dd16346b56ba5a2f