[libc] support fully OOT build (#101287)

[libc] support fully OOT build (#101287)

Fully OOT build along with SCUDO:
```
mkdir oot
cp -r cmake libc compiler-rt oot
cp ./llvm/cmake/modules/* ./oot/cmake/Modules/
cd oot
mkdir build
cd build
cmake ../libc -DLIBC_USE_NEW_HEADER_GEN=On -DLLVM_LIBC_FULL_BUILD=On -DLLVM_LIBC_FULL_BUILD=On -DLLVM_LIBC_INCLUDE_SCUDO=On -DCOMPILER_RT_BUILD_SCUDO_STANDALONE_WITH_LLVM_LIBC=On -DCOMPILER_RT_SCUDO_STANDALONE_BUILD_SHARED=Off -DCMAKE_CXX_COMPILER=clang++ -DCMAKE_C_COMPILER=clang -DLLVM_LIBC_COMPILER_RT_PATH=../compiler-rt/ -DCOMPILER_RT_STANDALONE_BUILD=On -GNinja -DLLVM_COMPILER_IS_GCC_COMPATIBLE=On -DLLVM_RUNTIMES_BUILD=On
```