newlib.mk: get NEWLIB_INCLUDE_DIR absolute and without trailing slash

It replaces

    make BOARD=iotlab-m3 info-debug-variable-NEWLIB_INCLUDE_DIR
    /usr/bin/../lib/gcc/arm-none-eabi/7.2.1/../../../../arm-none-eabi/include/

with

    make BOARD=iotlab-m3 info-debug-variable-NEWLIB_INCLUDE_DIR
    /usr/arm-none-eabi/include

Without trailing slash and without relative '..' everywhere.

It also uses `realpath` instead of `abspath` to support Mingw32.