linux-user/signal.c: Rename MC_* defines

linux-user/signal.c: Rename MC_* defines

The SPARC code in linux-user/signal.c defines a set of
MC_* constants. On some SPARC hosts these are also defined
by sys/ucontext.h, resulting in build failures:

linux-user/signal.c:2786:0: error: "MC_NGREG" redefined [-Werror]
 #define MC_NGREG 19

In file included from /usr/include/signal.h:302:0,
                 from include/qemu/osdep.h:86,
                 from linux-user/signal.c:19:
/usr/include/sparc64-linux-gnu/sys/ucontext.h:59:0: note: this is the location of the previous definition
 # define MC_NGREG __MC_NGREG

Rename all these constants to SPARC_MC_* to avoid the clash.

Cc: qemu-stable@nongnu.org
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Message-id: 1517318239-15764-1-git-send-email-peter.maydell@linaro.org