[PARISC] fix code to find libgcc

Sam broke this with

commit 1f2bfbd0
Author: Sam Ravnborg <sam@ravnborg.org>
Date:   Sat May 5 10:18:41 2012 +0200

    kbuild: link of vmlinux moved to a script

But we should be deriving the location of libgcc in the same way as all
the other archs, so fix by adding a LIBGCC variable which is evaluated
in the makefile

Signed-off-by: James Bottomley <JBottomley@Parallels.com>