x86/EFI: use less crude a way of generating the build ID

x86/EFI: use less crude a way of generating the build ID

Recent enough binutils (2.25 onwards) support --build-id also for
COFF/PE output, and hence we should use that in favor of the original
hack when possible.

This gets complicated by the linker requiring at least one COFF object
file to attach the .buildid section to. Hence the patch introduces a
buildid.ihex (in order to avoid introducing binary files into the repo)
which then gets converted to a binary minimal COFF object (no sections,
no symbols).

Also (to avoid both code fragment going out of sync) remove an unneeded
ALIGN() from xen.lds.S: Adding an equivalent of it to the .buildid
section would cause the _erodata symbol to become associated with the
wrong section again (see commit 0970299de5 ["x86/EFI + Live Patch:
avoid symbol address truncation"]). And it's pointless because the
alignment already gets properly set by the input section(s).

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Acked-by: Andrew Cooper <andrew.cooper3@citrix.com>