lib: libc: newlib: Make newlib nano variant optional

The newlib nano variant is currently enabled by default when
`CONFIG_NEWLIB_LIBC=y` and the selected toolchain-architecture
combination includes the newlib nano variant support, even if
`CONFIG_NEWLIB_LIBC_NANO` is not selected by the user.

When `CONFIG_NEWLIB_LIBC=y`, this results in the newlib nano variant
being selected for some architectures (e.g. ARC, ARM and RISC-V), while
the full variant is selected for the rest of the architectures.

The above behaviour is problematic because there exist functional
differences between the newlib full and nano variants (e.g. C99 format
modifiers such as `hh`, `ll`, `z`, `j` and `t` are not available in the
newlib nano variant), and this effectively leads to different level of
C standard support across different architectures when
`CONFIG_NEWLIB_LIBC=y`.

This commit fixes this problem by making the `CONFIG_NEWLIB_LIBC_NANO`
not `default y` and requiring its user to explicitly set this symbol to
`y` when they want to use the newlib nano variant.

Signed-off-by: Stephanos Ioannidis <stephanos.ioannidis@nordicsemi.no>