[compiler-rt] Fix check-builtins buildbot failures (#174443)
After #171941, there are two issues:
First, some buildbots that use the old-style build are failing at:
```
Target ${BUILTIN_LIB_TARGET_NAME} does not exist"
```
Example failure:
https://lab.llvm.org/buildbot/#/builders/139/builds/25097
...during CMake configure. This appears to be caused by mismatch between
the builtin library's _target_ name and the output name from
set_output_name. This reverts the change to BUILTIN_LIB_TARGET_NAME made
by #171941, but still use the output name for naming the .sources.txt
file used for configuring builtins tests.
Second, this speculatively fixes an issue caused by two builtins
libraries that are produced with the same name in different directories
because of `LLVM_ENABLE_PER_TARGET_RUNTIME_DIR` (e.g.
`lib/clang/22/lib/i386-unknown-linux-gnu/libclang_rt.builtins.a` and
`lib/clang/22/lib/x86_64-unknown-linux-gnu/libclang_rt.builtins.a`), and
thus the `.sources.txt` paths alias. This causes us to run the wrong
tests against one of the builtins libraries. The chosen fix is to store
the .sources.txt files in `get_compiler_rt_output_dir` (which takes
`LLVM_ENABLE_PER_TARGET_RUNTIME_DIR` into account) rather than
`CMAKE_BINARY_DIR`.
As a side-effect, this allows for the replacement of
`COMPILER_RT_TEST_BUILTINS_DIR` with a simpler boolean option
`COMPILER_RT_TEST_EXTERNAL_BUILTINS`.
Example failure:
https://lab.llvm.org/buildbot/#/builders/66/builds/24433
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