Prevent compile-time symlinks in kernel
So far, architecture specific headers for kernel were in
kernel/arch/$ARCH/include directory.
From kernel sources, they were referenced as arch/header.h.
For example, file kernel/arch/$ARCH/include/whatever.h
was included with <arch/whatever.h>.
To allow that, a symbolic link with name `arch' pointing
to the correct `include/' was created during compilation.
This change adds one arch/ directory and instead of
creating a symbolic link for each compilation, -I flag
was added to the compiler (the header mentioned above would
now reside in kernel/arch/$ARCH/include/arch/whatever.h).
The same goes for genarch/ includes.
This change would be followed by similar changes in userspace
and ABI. To overall goal is to simplify job of documenation
generators or IDEs that might have problems with the dynamically
created symbolic links.