[bios] Add bin-x86_64-pcbios build platform

Move most arch/i386 files to arch/x86, and adjust the contents of the
Makefiles and the include/bits/*.h headers to reflect the new
locations.

This patch makes no substantive code changes, as can be seen using a
rename-aware diff (e.g. "git show -M5").

This patch does not make the pcbios platform functional for x86_64; it
merely allows it to compile without errors.

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

src/arch/x86/include/pnpbios.h

@@ -0,0 +1,17 @@
+#ifndef _PNPBIOS_H
+#define _PNPBIOS_H
+
+/** @file
+ *
+ * PnP BIOS
+ *
+ */
+
+FILE_LICENCE ( GPL2_OR_LATER_OR_UBDL );
+
+/* BIOS segment address */
+#define BIOS_SEG 0xf000
+
+extern int find_pnp_bios ( void );
+
+#endif /* _PNPBIOS_H */