kbuild, modpost: fix unexpected non-allocatable section when cross compiling

The missing TO_NATIVE(sechdrs[i].sh_flags) was causing many
unexpected non-allocatable section warnings when cross-compiling
for an architecture with a different endianness.

Fix endianness of all the fields in the ELF header and
section headers, not just some of them so we are not
hit by this anohter time.

Signed-off-by: Anders Kaseorg <andersk@mit.edu>
Reported-by: Sean MacLennan <smaclennan@pikatech.com>
Tested-by: Sean MacLennan <smaclennan@pikatech.com>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>