arm64: Move handling of erratum 1418040 into C code

Instead of dealing with erratum 1418040 on each entry and exit,
let's move the handling to __switch_to() instead, which has
several advantages:

- It can be applied when it matters (switching between 32 and 64
  bit tasks).
- It is written in C (yay!)
- It can rely on static keys rather than alternatives

Signed-off-by: Marc Zyngier <maz@kernel.org>
Tested-by: Sai Prakash Ranjan <saiprakash.ranjan@codeaurora.org>
Reviewed-by: Stephen Boyd <swboyd@chromium.org>
Acked-by: Will Deacon <will@kernel.org>
Link: https://lore.kernel.org/r/20200731173824.107480-2-maz@kernel.org
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>