PowerPC: Add support for -msingle-pic-base

-msingle-pic-base is a new gcc option for ppc and
it reduces the size of my u-boot with 6-8 KB.
While at it, add -fno-jump-tables too to save a
few more bytes.

-msingle-pic-base will be in gcc 4.6, however
backported patches are available at
http://bugs.gentoo.org/show_bug.cgi?id=347281



Signed-off-by: Joakim Tjernlund <Joakim.Tjernlund@transmode.se>