[uaccess] Generalise librm's virt_offset mechanism for RISC-V

The virtual offset memory model used for i386-pcbios and x86_64-pcbios
can be generalised to also cover riscv32-sbi and riscv64-sbi.  In both
architectures, the 32-bit builds will use a circular map of the 32-bit
address space, and the 64-bit builds will use an identity map for the
relevant portion of the physical address space, with iPXE itself
placed in the negative (kernel) address space.

Generalise and document the virt_offset mechanism, and set it as the
default for both PCBIOS and SBI platforms.

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

src/core/virt_offset.c

@@ -0,0 +1,36 @@
+/*
+ * Copyright (C) 2025 Michael Brown <mbrown@fensystems.co.uk>.
+ *
+ * This program is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU General Public License as
+ * published by the Free Software Foundation; either version 2 of the
+ * License, or any later version.
+ *
+ * This program is distributed in the hope that it will be useful, but
+ * WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, write to the Free Software
+ * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA
+ * 02110-1301, USA.
+ *
+ * You can also choose to distribute this program under the terms of
+ * the Unmodified Binary Distribution Licence (as given in the file
+ * COPYING.UBDL), provided that you have satisfied its requirements.
+ */
+
+FILE_LICENCE ( GPL2_OR_LATER_OR_UBDL );
+
+#include <ipxe/uaccess.h>
+
+/**
+ * @file
+ *
+ * Virtual offset memory model
+ *
+ */
+
+PROVIDE_UACCESS_INLINE ( offset, phys_to_virt );
+PROVIDE_UACCESS_INLINE ( offset, virt_to_phys );