[JITLink][AArch32] Run all error unittests throug main entrypoints (#72091)

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()`.