Insert padding in __cxa_exception struct for compatibility

Similar to https://github.com/llvm/llvm-project/commit/f2a436058fcb, the
addition of __attribute__((__aligned__)) to _Unwind_Exception (in commit
b9616964) causes implicit padding to be inserted before the unwindHeader
field in __cxa_exception.

Applications attempt to get at the earlier fields in __cxa_exception, so
preserve the same negative offsets in __cxa_exception, by moving the
padding to the beginning of the struct.

The assumption here is that if the ABI is not aware of the padding
before unwindHeader and put the referenceCount/primaryException in
there, no padding should exist before unwindHeader.