Add picolibc compatibilty

[rben-dev]

This PR adds picolibc C standard library (https://github.com/picolibc/picolibc) compatibility. This is useful e.g. under Debian and some other distros where the RISC-V toolchain comes with a packaged picolibc but no packaged newlib for this target.

Under Debian trixie or Ubuntu jammy:

$ sudo apt install gcc-riscv64-unknown-elf picolibc-riscv64-unknown-elf

The PR should not break the regular original compilation with newlib as it is only activated when the environment variable USE_PICOLIBC=1 is exported.

The profiling script flow/eprof.py has been adapted to handle cases where source references from the debug symbols are not present in the toolchain (in this case the script skips these files).