Move definition of NOSTDINC_FLAGS below inclusion of arch Makefile, so
any arch specific settings to $(CC) takes effect before looking up the
compiler include directory.

The previous solution that replaced ':=' with '=' caused gcc to be
invoked one additional time for each directory visited.

This decreases kernel compile time with 0.1 second (3.6 -> 3.5 seconds) when
running make on a fully built kernel
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>