xtensa: don't use a12 in __xtensa_copy_user in call0 ABI

a12 is callee-saved register in xtensa call0 ABI, so a function must not
change it. The main unaligned copy loop of __xtensa_copy_user uses all
low-numbered registers, so a register must be spilled to avoid using a12
as a loop counter.

Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>