allow developer to select desired implementation of stdin, etc.

[gneverov]

picolibc_interface.c currently defines strong symbols for stdin, stdout, and stderr that use the implementation from pico_stdio. There is no way for the developer to override this behavior.

Use cases

• if picolibc was compiled with POSIX_IO, then picolibc already defines weak versions of stdin, etc.. Probably the developer intends to use this implementation, but it is silently overridden by the pico-sdk's implementation, which uses strong symbols.
• the developer might define their own symbols for stdin, etc. and doesn't want them overridden or in conflict with what pico-sdk defines.