Remove ${objtree}/include/asm/proc/ link

mkconfig links ${objtree}/include/asm/proc/ to
${srctree}/arch/${arch}/include/asm/proc-armv/. This seems to be a
remnant from the past. Ever since its introduction in 2003 it is used
only in ARM build and always links to same place, so let's simplify
the code, remove it and reference directly where needed.

Successful MAKEALL for ARM and PowerPC verified on Linux.

Signed-off-by: Vasili Galka <vvv444@gmail.com>