[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/include/bits/virt_offset.h

@@ -0,0 +1,15 @@
+#ifndef _BITS_VIRT_OFFSET_H
+#define _BITS_VIRT_OFFSET_H
+
+/** @file
+ *
+ * Dummy architecture-specific virtual address offset
+ *
+ * This file is included only if the architecture does not provide its
+ * own version of this file.
+ *
+ */
+
+FILE_LICENCE ( GPL2_OR_LATER_OR_UBDL );
+
+#endif /* _BITS_VIRT_OFFSET_H */