configure: disable MTTCG for MIPS guests

While debugging check-acceptance failures I found an instability in
the mips64el test case. Briefly the test case:

  retry.py -n 100 -c -- ./mips64el-softmmu/qemu-system-mips64el \
    -display none -vga none -serial mon:stdio \
    -machine malta -kernel ./vmlinux-4.7.0-rc1.I6400 \
    -cpu I6400 -smp 8 -vga std \
    -append "printk.time=0 clocksource=GIC console=tty0 console=ttyS0 panic=-1" \
    --no-reboot

Reports about a 9% failure rate:

  Results summary:
  0: 91 times (91.00%), avg time 5.547 (0.45 varience/0.67 deviation)
  -6: 9 times (9.00%), avg time 3.394 (0.02 varience/0.13 deviation)
  Ran command 100 times, 91 passes

When re-run with "--accel tcg,thread=single" the instability goes
away.

  Results summary:
  0: 100 times (100.00%), avg time 17.318 (249.76 varience/15.80 deviation)
  Ran command 100 times, 100 passes

Which seems to indicate there is some aspect of the MIPS MTTCG fixes
that has been missed. Ideally we would fix that but I'm afraid I don't
have time to investigate and am not super familiar with the
architecture anyway. In lieu of someone tracking down the failure lets
disable it for now.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Acked-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Acked-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Aleksandar Markovic <aleksandar.qemu.devel@gmail.com>
Cc: Aurelien Jarno <aurelien@aurel32.net>
Cc: Aleksandar Rikalo <aleksandar.rikalo@rt-rk.com>
Message-Id: <20200323161514.23952-7-alex.bennee@linaro.org>

configure

@@ -7887,7 +7887,7 @@ case "$target_name" in
     TARGET_SYSTBL_ABI=n32
   ;;
   mips64|mips64el)
-    mttcg="yes"
+    mttcg="no"
     TARGET_ARCH=mips64
     TARGET_BASE_ARCH=mips
     echo "TARGET_ABI_MIPSN64=y" >> $config_target_mak