target-i386: Ensure bit 10 on DR7 is never cleared

Bit 10 of DR7 is documented as always set to 1, so ensure that's
always the case.

Reviewed-by: Richard Henderson <rth@twiddle.net>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>

target-i386/bpt_helper.c

diff --git a/target-i386/bpt_helper.c b/target-i386/bpt_helper.c
index 23ce828..49472ea 100644
--- a/target-i386/bpt_helper.c
+++ b/target-i386/bpt_helper.c
@@ -85,6 +85,8 @@
     target_ulong old_dr7 = env->dr[7];
     int i;
 
+    new_dr7 |= DR7_FIXED_1;
+
     /* If nothing is changing except the global/local enable bits,
        then we can make the change more efficient.  */
     if (((old_dr7 ^ new_dr7) & ~0xff) == 0) {