LoongArch: Improve detection of valid TripleABI (#147952)
If the environment is considered to be the triple component as a whole,
so, including the object format, if any, and if that is the intended
behaviour, then the loongarch64 function `computeTargetABI()` should be
changed to not rely on `hasEnvironment()`, but, rather, to check if
there is a non-unknown environment set.
Without this change, using a (ideally valid) target of
loongarch64-unknown-none-elf, with a manually specified ABI of lp64s,
will result in a completely superfluous warning:
```
warning: triple-implied ABI conflicts with provided target-abi 'lp64s', using target-abi
```
