[llvm] [llvm-libgcc][CMake] Refactor llvm-libgcc (PR #65455)

[Fangrui Song via llvm-commits]

MaskRay wrote:

FWIW I did notice that the build had been broken (likely for a while) when I recently removed some `__{,de}register_frame_info*` and `__register_frame_table` in be91bd012123de835f64e10e677b24a7580b273c. Thank you for fixing the CMake.

```
-DLLVM_ENABLE_PROJECTS='clang;lld' -DLLVM_ENABLE_RUNTIMES='libcxx;libcxxabi;libunwind;llvm-libgcc;compiler-rt'
-DCOMPILER_RT_BUILTINS_HIDE_SYMBOLS=off  -DLLVM_LIBGCC_EXPLICIT_OPT_IN=on
```

This patch makes `libgcc` buildable but note that `llvm-libgcc` is not a target of the build directory
```
% ninja -C /tmp/out/custom libgcc
ninja: Entering directory `/tmp/out/custom'
ninja: error: unknown target 'libgcc
% ninja -C /tmp/out/custom llvm-libgcc
...
ninja: error: unknown target 'llvm-libgcc'
FAILED: runtimes/CMakeFiles/llvm-libgcc /tmp/out/custom/runtimes/CMakeFiles/llvm-libgcc
cd /tmp/out/custom/runtimes/runtimes-bins && /usr/bin/cmake --build /tmp/out/custom/runtimes/runtimes-bins/ --target llvm-libgcc --config Release
ninja: build stopped: subcommand failed.
% ninja -C /tmp/out/custom/runtimes/runtimes-bins/ libgcc  # good
% ninja -C /tmp/out/custom/runtimes/runtimes-bins/ llvm-libgcc
ninja: Entering directory `/tmp/out/custom/runtimes/runtimes-bins/'
ninja: error: unknown target 'llvm-libgcc'
```

Perhaps you can fix this as well.

https://github.com/llvm/llvm-project/pull/65455