Using "ld --oformat binary" for mbr.bin and usbdisk.bin seems to cause
segmentation faults on some versions of binutils (observed on Fedora
27).  Work around this problem by using ld to create an intermediate
ELF object, followed by objcopy (via the existing %.tmp -> %.bin rule)
to create the final binary.

Note that we cannot simply use a single-stage "objcopy -O binary"
since this will not process the relocation records for x86_64: see
commit 1afcccd ("[build] Do not use "objcopy -O binary" for objects
with relocation records").

Reported-by: Brent S <bts@square-r00t.net>
Signed-off-by: Michael Brown <mcb30@ipxe.org>

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>

Several of the assembly files in arch/i386/prefix were missed by the
automated relicensing tool due to missing licence declarations, code
dating back to the initial git revision, etc.  Manual review shows
that these files may be relicensed.

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

This reverts commit 844d088c51.

(including logical partitions).