[CMake] Enable LLVM_ENABLE_PER_TARGET_RUNTIME_DIR by default on Linux

This makes the default build closer to a -DLLVM_ENABLE_RUNTIMES=all build.
The layout is arguably superior because different libraries of target triples
are in different directories, similar to GCC/Debian multiarch.

When LLVM_DEFAULT_TARGET_TRIPLE is x86_64-unknown-linux-gnu,
`lib/clang/14.0.0/lib/libclang_rt.asan-x86_64.a`
becomes
`lib/clang/14.0.0/lib/x86_64-unknown-linux-gnu/libclang_rt.asan.a`.

Clang has been detecting both paths since 2018 (D50547).

---

Note: Darwin needs to be disabled. The hierarchy needs to be sorted out.
The current -DLLVM_DEFAULT_TARGET_TRIPLE=off state is like:
```
lib/clang/14.0.0/lib/darwin/libclang_rt.profile_ios.a
lib/clang/14.0.0/lib/darwin/libclang_rt.profile_iossim.a
lib/clang/14.0.0/lib/darwin/libclang_rt.profile_osx.a
```

Windows needs to be disabled: https://reviews.llvm.org/D107799?id=368557#2963311

Differential Revision: https://reviews.llvm.org/D107799

Author: MaskRay

llvm/CMakeLists.txt

@@ -671,7 +671,12 @@ endif()
 set(LLVM_TARGET_TRIPLE_ENV CACHE STRING "The name of environment variable to override default target. Disabled by blank.")
 mark_as_advanced(LLVM_TARGET_TRIPLE_ENV)
 
-set(LLVM_ENABLE_PER_TARGET_RUNTIME_DIR OFF CACHE BOOL
+if(CMAKE_SYSTEM_NAME MATCHES "Linux")
+  set(LLVM_ENABLE_PER_TARGET_RUNTIME_DIR_default ON)
+else()
+  set(LLVM_ENABLE_PER_TARGET_RUNTIME_DIR_default OFF)
+endif()
+set(LLVM_ENABLE_PER_TARGET_RUNTIME_DIR ${LLVM_ENABLE_PER_TARGET_RUNTIME_DIR_default} CACHE BOOL
   "Enable per-target runtimes directory")
 
 set(LLVM_PROFDATA_FILE "" CACHE FILEPATH