cmake: move specs compiler and linker flags to toolchain properties

Moving specs argument to compiler and linker properties so that the
compiler and linker in use can decide how the flags are mapped / handled
for the compiler and linker in use.

This avoids specifying `--specs=spec.picolibc` for clang which prints a
warning about an unused argument.

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