[NFCI][ELF] Merge AgainstSymbol and AgainstSymbolWithTargetVA
The former is just a special case of the latter, ignoring the expr and
always just using the addend. If we use R_ADDEND as expr (which
previously had no effect, and so was misleadingly R_ABS not R_ADDEND in
all but one use) then we don't need to maintain this as a separate case.
Aside from the internal Computed Kind, this just leaves MipsMultiGotPage
as a special case; the only difference between the other two Kind values
is what needsDynSymIndex returns.
Reviewers: MaskRay
Reviewed By: MaskRay
Pull Request: https://github.com/llvm/llvm-project/pull/150798