arm64: alternatives: use tpidr_el2 on VHE hosts

Now that KVM uses tpidr_el2 in the same way as Linux's cpu_offset in
tpidr_el1, merge the two. This saves KVM from save/restoring tpidr_el1
on VHE hosts, and allows future code to blindly access per-cpu variables
without triggering world-switch.

Signed-off-by: James Morse <james.morse@arm.com>
Reviewed-by: Christoffer Dall <cdall@linaro.org>
Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>

CC: bogus bogus bogus

Changes since v3:
 * Moved 'alternatives_applied' test all into C,
 * Made enable method static and dragged up before first-use.

Changes since v1:
 * cpu_copy_el2regs()'s 'have I been patched' test now always sets a register,
   just in case the compiler optimises out part of the logic.