modpost: Update 64k section support for binutils 2.18.50

modpost: Update 64k section support for binutils 2.18.50

Binutils 2.18.50 made a backwards-incompatible change in the way it
writes ELF objects with over 65280 sections, to improve conformance
with the ELF specification and interoperability with other ELF tools.
Specifically, it no longer adds 256 to section indices SHN_LORESERVE
and higher to skip over the reserved range SHN_LORESERVE through
SHN_HIRESERVE; those values are only considered special in the
st_shndx field, and not in other places where section indices are
stored.  See:

http://sourceware.org/bugzilla/show_bug.cgi?id=5900
http://groups.google.com/group/generic-abi/browse_thread/thread/e8bb63714b072e67/6c63738f12cc8a17

Signed-off-by: Anders Kaseorg <andersk@ksplice.com>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>