[Unwind] Harmonise exception class for EHABI spec.

EHABI defines the exception class as char[8] instead of uint64_t [1].
For ABI compatibility the ABI the definition needs to be updated.

[1] https://github.com/ARM-software/abi-aa/blob/main/ehabi32/ehabi32.rst#82language-independent-unwinding-types-and-functions

Reviewed By: manojgupta, MaskRay, #libunwind

Differential Revision: https://reviews.llvm.org/D109047