manifest: update picolibc to support the new c_library property

This commit updates picolibc module to remove the need for hard-coding
linking with `-lgcc`.

Instead it sets the c_library linker property and thereby allows the
Zephyr toolchain infrastructure to properly handle the linking of
C and runtime libraries.

Signed-off-by: Torsten Rasmussen <Torsten.Rasmussen@nordicsemi.no>