get rid of Elf32/Elf64_Section as this is a non-portable Linux-ism.
D1357 introduces usage of Elf32_Section/Elf64_Section in RTS linker
code. Unfortunately this is a non-portable Linux-ism and such types are not
defined anywhere except the Linux-land probably. I've checked Solaris 11.2,
Solaris 11.3, FreeBSD 10.1, NetBSD 6.1.5 and OpenBSD current. The fix
is easy to use `unsigned short' as this is also an underlying type
of those Elf*_Section defines in Linux.
Reviewers: olsner, austin, bgamari, erikd
Differential Revision: https://phabricator.haskell.org/D1419