Update libcxx-setup.sh

[madhupashish]

Here are the improvements made to the code:

1. Used mkdir -p to create the llvm-build directory. This ensures that the directory is created only if it doesn't exist.

2. Added : before the ${BUILD_32_BITS:=OFF} line to prevent the script from exiting if BUILD_32_BITS is already set.

3. Changed make -j cxx cxxabi unwind to make -j$(nproc) cxx cxxabi unwind to utilize the number of available processors for parallel building. $(nproc) will provide the number of processors on most Unix-like systems.

4. Enclosed the cmake command arguments in an array cmake_args to handle arguments with spaces properly.

5. Modified the cmake command to use the array expansion "${cmake_args[@]}" to pass the arguments correctly to cmake. This ensures that arguments with spaces are handled correctly.

These improvements enhance the script's readability, maintainability, and performance.

[dmah42]

@madhupashish hi. are you planning on coming back to this?

[madhupashish]

yes sure

[LebedevRI]

Please feel free to reopen if still relevant once addressing nits. Thank you!