llvmpipe: disable VSX in ppc due to LLVM PPC bug

This patch disables the use of VSX instructions, as they cause some
piglit tests to fail

For more details, see: https://llvm.org/bugs/show_bug.cgi?id=25503#c7

With this patch, ppc64le reaches parity with x86-64 as far as piglit test
suite is concerned.

v2:
- Added check that we have at least LLVM 3.4
- Added the LLVM bug URL as a comment in the code

v3:

- Only disable VSX if Altivec is supported, because if Altivec support
is missing, then VSX support doesn't exist anyway.

- Change original patch description.

Signed-off-by: Oded Gabbay <oded.gabbay@gmail.com>
Cc: "11.0" <mesa-stable@lists.freedesktop.org>
Reviewed-by: Jose Fonseca <jfonseca@vmware.com>
Reviewed-by: Roland Scheidegger <sroland@vmware.com>

src/gallium/auxiliary/gallivm/lp_bld_misc.cpp

@@ -536,6 +536,15 @@ lp_build_create_jit_compiler_for_module(LLVMExecutionEngineRef *OutJIT,
 
 #if defined(PIPE_ARCH_PPC)
    MAttrs.push_back(util_cpu_caps.has_altivec ? "+altivec" : "-altivec");
+#if HAVE_LLVM >= 0x0304
+   /*
+    * Make sure VSX instructions are disabled
+    * See LLVM bug https://llvm.org/bugs/show_bug.cgi?id=25503#c7
+    */
+   if (util_cpu_caps.has_altivec) {
+      MAttrs.push_back("-vsx");
+   }
+#endif
 #endif
 
    builder.setMAttrs(MAttrs);