[libunwind] Fix CIE v1 return address parsing

 - For CIE version 1 (e.g. in DWARF 2.0.0), the return_address_register
   field is a ubyte [0..255].

 - For CIE version 3 (e.g. in DWARF 3), the field is instead a ULEB128
   constant.

Previously, libunwind accepted a CIE version of 1 or 3, but always
parsed the field as ULEB128.

Clang always outputs CIE version 1 into .eh_frame. (It can output CIE
version 3 or 4, but only into .debug_frame.)

Differential Revision: https://reviews.llvm.org/D83741