[Runtimes] Rework and remove default runtimes build

[jhuber6]

Summary:
Currently the runtimes builds works by creating separate CMake projects
that build the respetive runtime. Right now we have a separate handling
for the 'default' target and manually specific runtimes via the
-DLLVM_RUNTIME_TARGETS option.

This patch changes the behavior to put all runtimes through the
LLVM_RUNTIME_TARGETS pipeline. The old "default" argument is now a
shorthand for LLVM_DEFAULT_TARGET_TRIPLE and corresponds to a sane
default.

In practical terms, this means the old runtimes-bins directory will
now be runtimes-x86_64-unknown-linux-gnu-bins for the majority of
users. We will not have check-<name> targets, but I have made
a top-level target that invokes all of the enabled targets check lines
to keep this backward compatible. So ninja check-cxx will still work.

There's likely some edge cases I missed here, but it seems to work in
the typical case for me. We'll see what CI thinks of this.

Fixes: #98897

[mgorny]

Oh, that's llvm/runtimes and not top-level runtimes, so I guess it won't affect Gentoo.

[jhuber6]

Oh, that's llvm/runtimes and not top-level runtimes, so I guess it won't affect Gentoo.

Yes, ideally this change doesn't affect anyone that isn't depending on the name of directories in the build tree. Fundamentally I'm hoping to make our runtimes infrastructure more predictable by removing this weird disconnect we have between the generic and specific targets. More or less, this change is equivalent to the user putting -DRUNTIME_TARGETS=${LLVM_DEFAULT_TARGET_TRIPLE}. I want to keep the functionality of default because it's very important that things like cache files can target the user's default system.

I added the check targets to give an interface that looks the same as what we have now for the majority of users.

[Meinersbur]

[nit] unrelated change

[jhuber6]

Ping, I'm hoping this change is NFC in practice. Also I forgot to mention that this solves #98897.

[petrhosek]

Ping, I'm hoping this change is NFC in practice. Also I forgot to mention that this solves #98897.

Unfortunately it's not, this change would break builds on Apple platforms because they don't use target triple for building runtimes, that's the reason why the default target was originally introduced. I have an idea for addressing this but that's going to require more significant changes to the build and Clang.

[jhuber6]

Ping, I'm hoping this change is NFC in practice. Also I forgot to mention that this solves #98897.

Unfortunately it's not, this change would break builds on Apple platforms because they don't use target triple for building runtimes, that's the reason why the default target was originally introduced. I have an idea for addressing this but that's going to require more significant changes to the build and Clang.

Interesting, could you elaborate on this? As far as I understand, each target has a default target triple. The main difference here is the directory naming and the fact that it passes --target= to the internal CMake job. Is this the part that causes issues? Whatever solution we end up with it will be very important that users can use default or similar to target the user's default environment, because we'd want to provide build scripts and cache files that don't require users to fully understand the concept of an LLVM triple.