[PECOFF] Add DISCARDABLE characteristic to .reloc section

This patch sets the IMAGE_SCN_MEM_DISCARDABLE characteristic to the base
relocations section in order to match MS PECOFF specification.

Patch by Ron Ofir.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@190798 91177308-0d34-0410-b5e6-96231b3b80d8