tcg: fix 16-byte vector operations detection

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: db43267 "tcg: Add generic vector expanders"
Signed-off-by: Laurent Vivier <lvivier@redhat.com>
Reviewed-by: Miroslav Rezanina <mrezanin@redhat.com>
Message-id: 20180328133152.24623-1-lvivier@redhat.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>