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>
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