[XTENSA] Clean up elf-gregset.

[XTENSA] Clean up elf-gregset.

Remove additional registers from the ELF gregset structure that
are only used by the kernel or are not required or invalid in
user-space. The ar registers are always aligned to a windowbase
value of 0, and the WB register is always assumed to be 0.
Increase the size of the structure to 128 entries. This will
provide enough space in future.

Signed-off-by: Chris Zankel <chris@zankel.net>