mips: fix build of vmlinux.lds

Commit 51b563fc ("arm, cris, mips,
sparc, powerpc, um, xtensa: fix build with bash 4.0") removed a few
CPPFLAGS with vital include paths necessary to build vmlinux.lds
on MIPS, and moved the calculation of the 'jiffies' symbol
directly to vmlinux.lds.S but forgot to change make ifdef/... to
cpp macros.

Signed-off-by: Manuel Lauss <manuel.lauss@gmail.com>
[sam: moved assignment of CPPFLAGS arch/mips/kernel/Makefile]
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Acked-by: Dmitri Vorobiev <dmitri.vorobiev@movial.com>