Fix truncation of relocation types in Support/ELF.h

This is a follow-up to r171845, which fixes the same issue in the Support code.
Only targets with >256 relocations (principally AArch64) should be affected.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@173151 91177308-0d34-0410-b5e6-96231b3b80d8