Fix logic of pthread type sizes to match C headers.

One fix for X32... with a few more to come yet.

Updates definition to match C headers:
```
#ifdef __x86_64__
# if __WORDSIZE == 64
#  define __SIZEOF_PTHREAD_ATTR_T 56
#  define __SIZEOF_PTHREAD_MUTEX_T 40
#  define __SIZEOF_PTHREAD_MUTEXATTR_T 4
#  define __SIZEOF_PTHREAD_COND_T 48
#  define __SIZEOF_PTHREAD_CONDATTR_T 4
#  define __SIZEOF_PTHREAD_RWLOCK_T 56
#  define __SIZEOF_PTHREAD_RWLOCKATTR_T 8
#  define __SIZEOF_PTHREAD_BARRIER_T 32
#  define __SIZEOF_PTHREAD_BARRIERATTR_T 4
# else
#  define __SIZEOF_PTHREAD_ATTR_T 32
#  define __SIZEOF_PTHREAD_MUTEX_T 32
#  define __SIZEOF_PTHREAD_MUTEXATTR_T 4
#  define __SIZEOF_PTHREAD_COND_T 48
#  define __SIZEOF_PTHREAD_CONDATTR_T 4
#  define __SIZEOF_PTHREAD_RWLOCK_T 44
#  define __SIZEOF_PTHREAD_RWLOCKATTR_T 8
#  define __SIZEOF_PTHREAD_BARRIER_T 20
#  define __SIZEOF_PTHREAD_BARRIERATTR_T 4
# endif
#else
# define __SIZEOF_PTHREAD_ATTR_T 36
# define __SIZEOF_PTHREAD_MUTEX_T 24
# define __SIZEOF_PTHREAD_MUTEXATTR_T 4
# define __SIZEOF_PTHREAD_COND_T 48
# define __SIZEOF_PTHREAD_CONDATTR_T 4
# define __SIZEOF_PTHREAD_RWLOCK_T 32
# define __SIZEOF_PTHREAD_RWLOCKATTR_T 8
# define __SIZEOF_PTHREAD_BARRIER_T 20
# define __SIZEOF_PTHREAD_BARRIERATTR_T 4
#endif
```

I would suspect this would also fix LP64-but-not-x86_64 targets also.

Thanks.