[Headers][mips] Add `__attribute__((__mode__(__unwind_word__)))` to the...

[Headers][mips] Add `__attribute__((__mode__(__unwind_word__)))` to the _Unwind_Word / _Unwind_SWord definitions

The rationale of this change is to fix _Unwind_Word / _Unwind_SWord
definitions for MIPS N32 ABI. This ABI uses 32-bit pointers,
but _Unwind_Word and _Unwind_SWord types are eight bytes long.

 # The __attribute__((__mode__(__unwind_word__))) is added to the type
   definitions. It makes them equal to the corresponding definitions used
   by GCC and allows to override types using `getUnwindWordWidth` function.
 # The `getUnwindWordWidth` virtual function override in the `MipsTargetInfo`
   class and provides correct type size values.

Differential revision: https://reviews.llvm.org/D58165

llvm-svn: 353965