PPC: Fix missing TRACE exception

This patch fixes bug 1031698 :
https://bugs.launchpad.net/qemu/+bug/1031698

If we look at the (truncated) translation of the conditional branch
instruction in the test submitted in the bug post, the call to the
exception helper is missing in the "bne-false" chunk of translated
code :

IN:
bne-    0x1800278

OUT:
0xb544236d:  jne    0xb5442396

0xb5442373:  mov    %ebp,(%esp)
0xb5442376:  mov    $0x44,%ebx
0xb544237b:  mov    %ebx,0x4(%esp)
0xb544237f:  mov    $0x1800278,%ebx
0xb5442384:  mov    %ebx,0x25c(%ebp)
0xb544238a:  call   0x827475a
                     ^^^^^^^^^^^^^^^^^^

0xb5442396:  mov    %ebp,(%esp)
0xb5442399:  mov    $0x44,%ebx
0xb544239e:  mov    %ebx,0x4(%esp)
0xb54423a2:  mov    $0x1800270,%ebx
0xb54423a7:  mov    %ebx,0x25c(%ebp)

Indeed, gen_exception(ctx, excp) called by gen_goto_tb (called by
gen_bcond) changes ctx->exception's value to excp's :

gen_bcond()
{
  gen_goto_tb(ctx, 0, ctx->nip + li - 4);
  /* ctx->exception value is POWERPC_EXCP_BRANCH */

  gen_goto_tb(ctx, 1, ctx->nip);
  /* ctx->exception now value is POWERPC_EXCP_TRACE */
}

Making the following gen_goto_tb()'s test false during the second call :

if ((ctx->singlestep_enabled &
    (CPU_BRANCH_STEP | CPU_SINGLE_STEP)) &&
    ctx->exception == POWERPC_EXCP_BRANCH /* false...*/) {
         target_ulong tmp = ctx->nip;
         ctx->nip = dest;
         /* ... and this is the missing call */
         gen_exception(ctx, POWERPC_EXCP_TRACE);
         ctx->nip = tmp;
}

So the patch simply adds the missing matching case, fixing our problem.

Signed-off-by: Julio Guerra <guerr@julio.in>
Signed-off-by: Alexander Graf <agraf@suse.de>