[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
[PECOFF] Take into account all sections when setting size fields in the PE header

This patch changes lld to go through all sections while calculating the size
for SizeOfCode, SizeOfInitializedData and SizeOfUninitializedData fields in the
PE header, instead of using only a small set of hard-coded sections.

This only really changes SizeOfInitializedData which didn't include .reloc
section before this patch.

Patch by Ron Ofir.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@190799 91177308-0d34-0410-b5e6-96231b3b80d8