Support selection of different printf implementations

[voltur01]

Support selection of different printf implementations provided by picolibc.

picolibc provides a mechanism based on --defsym linker option to select which printf implementation to use, see https://github.com/picolibc/picolibc/blob/main/doc/printf.md

This works with GNU LD, however does not work with LLD because of llvm/llvm-project#52, there is a specific reference there llvm/llvm-project#52

We need to either help progress the LLD change above or invent something different as a workaround.

[keith-packard]

If you split out the printf from scanf selection, at least you'd avoid pulling in the scanf code with this?