cmake: Fix setting of -nostdinc

We where defining the variable NOSTDINC_F after we tried to use it.
Move the definition before the first reference fixes things.  When
-nostdinc now enabled we need to explicitly add the compiler include
path for x86_64 based builds (x86_64 and ARCH_POSIX).

Signed-off-by: Kumar Gala <kumar.gala@linaro.org>
2 files changed