[libc][NFC] modularize malloc

[SchrodingerZhu]

modularize malloc implementations.

[lntue]

I don't think you need to introduce an adhoc way to fix target name. Since you already refactoring all the choices to their own folder, in the allocator's cmake, you can add_subdirectory and alias libc.src.stdlib.* to libc.src.stdlib.allocator.<option>.* based on the option.

[SchrodingerZhu]

@lntue ideally that is the correct thing to do. however, I failed to alias an external object.

• It is rejected by explicit checking
• After relaxing the check, the generator expression is unhappy

[lntue]

@lntue ideally that is the correct thing to do. however, I failed to alias an external object.

• It is rejected by explicit checking
• After relaxing the check, the generator expression is unhappy

Do you mind making a draft using alias that I can try and take a look at the errors?