[XTENSA] Clean up stat structs.

Avoid using typedefs for stat fields.
Make stat64.st_blocks an unsigned long long to avoid endian-specific
padding with 32-bit values.
Clean up signed vs. unsigned and int vs. long types to be consistent
with other uses of these values.

Signed-off-by: Bob Wilson <bob.wilson@acm.org>
Signed-off-by: Chris Zankel <chris@zankel.net>