[ELF] - Added support of linkerscript's "/DISCARD/" for --emit-relocs

Previously LLD crashed on on provided testcases because "/DISCARD/" was
not supported. Patch implements that.

After this I think there is no known issues with --emit-relocs implementation
required for linux kernel linking.

Differential revision: https://reviews.llvm.org/D29273

llvm-svn: 295488