tcg: fix 16-byte vector operations detection

configure tries to detect if the compiler
supports 16-byte vector operations.

As stated in the comment of the detection
program, there is a problem with the system
compiler on GCC on Centos 7.

This program doesn't actually detect the problem
with GCC on RHEL7 on PPC64LE (Red Hat 4.8.5-28).

This patch updates the test to look more like
it is in QEMU helpers, and now detects the problem.

The error reported is:

  CC      ppc64-softmmu/accel/tcg/tcg-runtime-gvec.o
  ..//accel/tcg/tcg-runtime-gvec.c: In function ‘helper_gvec_shl8i’:
  ../accel/tcg/tcg-runtime-gvec.c:558:26: internal compiler error: in emit_move_insn, at expr.c:3495
           *(vec8 *)(d + i) = *(vec8 *)(a + i) << shift;
                            ^
Fixes: db432672 "tcg: Add generic vector expanders"
Signed-off-by: NLaurent Vivier <lvivier@redhat.com>
Reviewed-by: NMiroslav Rezanina <mrezanin@redhat.com>
Message-id: 20180328133152.24623-1-lvivier@redhat.com
Signed-off-by: NPeter Maydell <peter.maydell@linaro.org>

configure

@@ -5054,6 +5054,14 @@ static S2 c2;
 static S4 c4;
 static S8 c8;
 static int i;
+void helper(void *d, void *a, int shift, int i);
+void helper(void *d, void *a, int shift, int i)
+{
+  *(U1 *)(d + i) = *(U1 *)(a + i) << shift;
+  *(U2 *)(d + i) = *(U2 *)(a + i) << shift;
+  *(U4 *)(d + i) = *(U4 *)(a + i) << shift;
+  *(U8 *)(d + i) = *(U8 *)(a + i) << shift;
+}
 int main(void)
 {
   a1 += b1; a2 += b2; a4 += b4; a8 += b8;