arm64: don't call C code with el0's fp register

arm64: don't call C code with el0's fp register

On entry from el0, we save all the registers on the kernel stack, and
restore them before returning. x29 remains unchanged when we call out
to C code, which will store x29 as the frame-pointer on the stack.

Instead, write 0 into x29 after entry from el0, to avoid any risk of
tracing into user space.

Signed-off-by: James Morse <james.morse@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>
(cherry picked from commit 49003a8d6b35e128ef5e51433e60e783a46fbe5f)
Signed-off-by: Alex Shi <alex.shi@linaro.org>