xen/build: put image header into a separate section

xen/build: put image header into a separate section

So it can be explicitly placed ahead of the rest of the .text content
in the linker script (and thus the resulting image). This is a
prerequisite for further work that will add a catch-all to the text
section (.text.*).

Note that placement of the sections inside of .text is also slightly
adjusted to be more similar to the position found in the default GNU
ld linker script.

The special handling of the object file containing the header data as
the first object file passed to the linker command line can also be
removed.

While there also remove the special handling of efi/ on x86. There's
no need for the resulting object file to be passed in any special
order to the linker.

Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Julien Grall <jgrall@amazon.com>