don't pass r12 as reference

don't pass r12 as reference

r12 and x16 are of different sizes; when passing r12 as a reference to
do_trap_hypercall on arm64, we end up dereferencing it as a pointer to a
64bit value, but actually it isn't.

Instead, use a temporary variable to pass r12, and write back the result
after the call to do_trap_hypercall.

CID: 1457708
Signed-off-by: Stefano Stabellini <sstabellini@kernel.org>
Reviewed-by: Julien Grall <julien.grall@linaro.org>