sh: Add -mno-fdpic to default flags.

Presently the --fdpic specifier and the --isa matching clash when
building with FDPIC toolchains. As we have no interest in building the
kernel with --fdpic in the first place, always try to add in -mno-fdpic
to the default flags.

Signed-off-by: Paul Mundt <lethal@linux-sh.org>