target-cris/opcode-cris.h: rename REG_PC/SP to CRIS_REG_PC/SP

target-cris/opcode-cris.h: rename REG_PC/SP to CRIS_REG_PC/SP

The REG_PC constant used in opcode-cris.h can clash with a
similar define in system include files. In particular the
Ubuntu Lucid SPARC signal.h will define REG_PC, and since
qemu-common.h now includes signal.h this was causing compile
failures. Rename the constants to avoid this issue.
(NB that REG_SP is not actually used within QEMU.)

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Edgar E. Iglesias <edgar.iglesias@gmail.com>