Reading implicit addend from a relocation site doesn't require a complete
`LinkGraph` edge. The operation is independent from `TargetSymbol`,
but constructing an `Edge` instance required one. This patch fixes the
inconsistency and simplifies some setup code from the error unittests.
Furthermore this patch prepares for the `Arm`/`Thumb`/`Data` helper
functions to be turned into implementation details. Exposing them in the
API causes unfortunate inconsistencies that we don't want to error-check
all the time, e.g. passing `Thumb_Call` to `readAddendArm()`.
This test checks for error paths in relocation dependent functions of readAddend and applyFixup. It is useful to check these to avoid unexpected assert errors. Currently opcode errors are triggered in most of the cases in AArch32 but there might be further checks to look for in the future. Different backends can also implement a similar test.
