[build] Fix building with newer binutils

[build] Fix building with newer binutils

Newer versions of the GNU assembler (observed with binutils 2.41) will
complain about the ".arch i386" in files assembled with "as --64",
with the message "Error: 64bit mode not supported on 'i386'".

In files such as stack.S that contain no instructions to be assembled,
the ".arch i386" is redundant and may be removed entirely.

In the remaining files, fix by moving ".arch i386" below the relevant
".code16" or ".code32" directive, so that the assembler is no longer
expecting 64-bit instructions to be used by the time that the ".arch
i386" directive is encountered.

Reported-by: Ali Mustakim <alim@forwardcomputers.com>
Signed-off-by: Michael Brown <mcb30@ipxe.org>