tcg/sparc: Use the proper compilation flags for 32-bit

We have required a v9 cpu since 9b9c37c364.
However, the flags we were using did not reliably enable v8plus, which
meant that the compiler didn't know it could inline 64-bit atomics.

Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Signed-off-by: Richard Henderson <rth@twiddle.net>

configure

@@ -1213,12 +1213,12 @@ case "$cpu" in
            LDFLAGS="-m64 $LDFLAGS"
            ;;
     sparc)
-           LDFLAGS="-m32 $LDFLAGS"
-           CPU_CFLAGS="-m32 -mcpu=ultrasparc"
+           CPU_CFLAGS="-m32 -mv8plus -mcpu=ultrasparc"
+           LDFLAGS="-m32 -mv8plus $LDFLAGS"
            ;;
     sparc64)
-           LDFLAGS="-m64 $LDFLAGS"
            CPU_CFLAGS="-m64 -mcpu=ultrasparc"
+           LDFLAGS="-m64 $LDFLAGS"
            ;;
     s390)
            CPU_CFLAGS="-m31"