target-i386: Fix eflags.TF/#DB handling of syscall/sysret insns

The syscall and sysret instructions behave a bit differently:
TF is checked after the instruction completes.
This allows the o/s to disable #DB at a syscall by adding TF to FMASK.
And then when the sysret is executed the #DB is taken "as if" the
syscall insn just completed.

Signed-off-by: Doug Evans <dje@google.com>
Message-Id: <94eb2c0bfa1c6a9fec0543057483@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>