There were no reason to mess around with CC, AS and LD.
Fixing this up avoided duplicated option for ld.

A small fixlet were needed in boot/Makefile which assumed
that CC were modified.
Signed-off-by: NSam Ravnborg <sam@ravnborg.org>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Signed-off-by: NIngo Molnar <mingo@elte.hu>
Signed-off-by: NThomas Gleixner <tglx@linutronix.de>