Fix attribute((mode([word|unwind_word]))) for x32

Fix attribute((mode([word|unwind_word]))) for x32

Patch by H.J. Lu

```
typedef unsigned int gcc_word __attribute__((mode(word)));
```
and

```
typedef unsigned int gcc_unwind_word __attribute__((mode(unwind_word)));
```
define the largest unsigned integer types which can be stored in a
general purpose register, which may not be the pointer type.  For x32,
they aren't pointer nor unsigned long.  We should

1. Make getUnwindWordWidth and getRegisterWidth virtual,
2. Override them for x32, similar to hasInt128Type.
3. Use getRegisterWidth for __attribute__((mode(word)));

This fixes PR 24706.

Reviewers: rnk

Differential Revision: http://reviews.llvm.org/D16779

llvm-svn: 259383