build: sys: Define NORMAL_CPP variable, to be used in 'files*'

[olce]

Summary

Define the NORMAL_CPP Makefile variable to the prefix to use to run the
preprocessor with appropriate flags as they are passed for compilation
of normal C files. It is to be used in preference to CPP, which does
not include '-I' or '-D' directives.

Note that, in contrast with NORMAL_C, NORMAL_CPP is never directly
produced by config(8) in the final kernel Makefile, as it does not (need
to) know how to just preprocess files.