makefiles/libc/newlibc: allow toolchains with nano version only
makefiles/libc/newlibc: allow toolchains with nano version only

The check for `newlib-nano` assumes that a toolchain which provides `newlib-nano` provides both the normal version and the nano version of the `newlib`.The nano version of `newlib.h` is therefore stored in its own include directory. This is the case for toolchains which allow to use both the normal and the nano version, e.g. for ARM and RISC-V.
However, if the toolchain provides `newlib_nano` but only allows the use of the nano version, it will only have the nano version of `newlib.h` and no separate directory for it, e.g. for ESP32.
To still be able to use such toolchains with `newlib_nano`, the check is changed so that the setting of the `-isystem` option depends on the existence of the separate directory.
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