arm64/vmm: Teach the switcher about new registers

To support booting the kernel in EL2 some of the EL0 and EL1 registers
are changed to point to an EL2 version. To get access to the EL0/EL1
version of these registers we need to use the new EL02 and EL12
registers, e.g. to access elr_el1 from the host we would use elr_el12.

Add macros that can be different on VHE vs non-VHE code so these
registers can be accessed in the correct way.

Sponsored by: Arm Ltd
Differential Revision: https://reviews.freebsd.org/D46078