newlib.mk: Windows/MSYS Bash support: use realpath instead of abspath

Some versions of Mingw32 abspath implementation has trouble working with
windows formatted path.

    $(abspath "C:/A/B") returns "/C/CUR/DIR/C:/A/B" instead of "/C/A/B"

relpath does not have this problem, it does additional symlink resolution but is
not a problem.
Note: on windows it does not remove the trailing `/`.

https://github.com/zephyrproject-rtos/zephyr/issues/2061#issuecomment-331635063

Patched in

https://github.com/zephyrproject-rtos/zephyr/commit/941059c69f9488613775a1a8802a099447cdea4c