arm64: remove __switch_data object from head.S

This removes the confusing __switch_data object from head.S,
and replaces it with standard PC-relative references to the
various symbols it encapsulates.

Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
Tested-by: Mark Rutland <mark.rutland@arm.com>
Reviewed-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Signed-off-by: Will Deacon <will.deacon@arm.com>