[riscv] Hold virtual address offset in the thread pointer register

[riscv] Hold virtual address offset in the thread pointer register

iPXE does not make use of any thread-local storage.  Use the otherwise
unused thread pointer register ("tp") to hold the current value of
the virtual address offset, rather than using a global variable.

This ensures that virt_offset can be made valid even during very early
initialisation (when iPXE may be executing directly from read-only
memory and so cannot update a global variable).

Signed-off-by: Michael Brown <mcb30@ipxe.org>