[yaml2obj][ELF] Add support for symbol indexes greater than SHN_LORESERVE

Right now Symbols must be either undefined or defined in a specific
section. Some symbols have section indexes like SHN_ABS however. This
change adds support for outputting symbols that have such section
indexes.

Patch by Jake Ehrlich

Differential Revision: https://reviews.llvm.org/D37391

llvm-svn: 312745