LoongArch: prevent building duplicate "default" multilib.

[scylaac]

[xry111]

I don't think it's the "correct" approach.

We should define MULTILIB_DEFAULTS for this purpose, instead of introduce those hacks. More serious problem is that the hacks may affect all targets, and we don't have resources to test all of them.

[xry111]

See #72.

[scylaac]

We should define MULTILIB_DEFAULTS for this purpose, instead of introduce those hacks. More serious problem is that the hacks may affect all targets, and we don't have resources to test all of them.

I believe the problem is not so serious. Here's my reasoning:

1. config-ml.in can be modified for a new backend.

2. The change made to config-ml.in's architecture-independent part is not a desperate "hack" (I guess you can say that the entire multilib thing is also pretty hacky :) ). It simply provides a new choice for the ${host}s to manage their multilib layout: Initially, ml_toplevel_p will not be set to yes unless with_multisubdir is empty (indicating that a top-level default lib is being configured), thus it's not possible to run into the new code path unless the case ${host} clause have made an explicit assignment to with_multisubdir (which is currently only done by loongarch, and will certainly not happen for any architectures that want to code their multilib support using the "canonical" approach). Any ${host} type can make this assignment to indicate which multilib build is a duplicate of the "default" library (which is the one being configured). Other architecture-specific adjustments are also needed, but as you can see in this patch, it's not much nor too complex.

Although setting MULTILIB_DEFAULT is conventional, it's not really compatible with our current design:

1. Implementing LA's architecture-specific option-handling logic with only C/C++ functions and $(tm_defines) macros (which provides better readability and flexibility to add more options). The job of MULTILIB_DEFAULT is now done by DRIVER_SELF_SPECS.

2. Making multilib directory layout symmetric (and constant across compilers with different default ABI).

[xry111]

2. Making multilib directory layout symmetric (and constant across compilers with different default ABI).

It will still be symmetric and constant using MULTILIB_DEFAULT. MULTILIB_DEFAULT has no effect on -print-multi-os-directory. It only affects -print-multi-lib, which is only used by gcc building process.

At now I think we should minimize the danger causing a regression in other platforms. Annoying the maintainer of an existing port is really bad in the reviewing process of a new port! :)

But if you guys can ensure that this won't break anything already existed (and convince those upstream maintainers to believe it [^1]), I can live with it (I don't really use multilib, even on x86_64).

[^1]: This is kind of like sending a paper: even if all of your logical sounds, the reviewer may still do not believe you! :)