stat on /dev/null should indicate S_IFCHR

[mikbras]

The following libc test fails because stat on /dev/null should set S_IFCHR bit in st_mode.

$ make one TEST=/src/functional/stat.exe
/home/mike/mystikos/build/bin/myst exec-sgx rootfs --memory-size=256m /src/functional/stat.exe
src/functional/stat.c:28: S_ISCHR(st.st_mode) failed:
Makefile:66: recipe for target 'one' failed
make: *** [one] Error 1