Simplify the handling of pcrel relocations on ELF. Now we do the right thing

Simplify the handling of pcrel relocations on ELF. Now we do the right thing
for all symbol differences and can drop the old EmitPCRelSymbolValue
method.

This also make getExprForFDESymbol on ELF equal to the one on MachO, and it
can be made non-virtual.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@130634 91177308-0d34-0410-b5e6-96231b3b80d8