[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/bootsector.h

@@ -0,0 +1,14 @@
+#ifndef _BOOTSECTOR_H
+#define _BOOTSECTOR_H
+
+/** @file
+ *
+ * x86 bootsector image format
+ */
+
+FILE_LICENCE ( GPL2_OR_LATER_OR_UBDL );
+
+extern int call_bootsector ( unsigned int segment, unsigned int offset,
+			     unsigned int drive );
+
+#endif /* _BOOTSECTOR_H */