Allow user to disable implementation of libc functions

[niansa]

The user might want to load their own libc implementation including functions like memset and printf. Currently this is impossible because ch32v003fun already defines these functions. There should be an option to not define these function or at least to define them with alternative names.

This would also require UART and DEBUG printing to be decoupled from stdio so the user would still be able to use those. See: #373