[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>

src/core/uaccess.c

@@ -34,3 +34,7 @@ FILE_LICENCE ( GPL2_OR_LATER_OR_UBDL );
 /* Flat address space user access API */
 PROVIDE_UACCESS_INLINE ( flat, phys_to_virt );
 PROVIDE_UACCESS_INLINE ( flat, virt_to_phys );
+
+/* Virtual address offset user access API */
+PROVIDE_UACCESS_INLINE ( offset, phys_to_virt );
+PROVIDE_UACCESS_INLINE ( offset, virt_to_phys );