test: Adjust for clang's removal of __builtin_shuffle

__builtin_shuffle was removed in clang 5.0.

Build log says:
test/utils-prng.c:207:27: error: use of unknown builtin '__builtin_shuffle' [-Wimplicit-function-declaration]
            randdata.vb = __builtin_shuffle (randdata.vb, bswap_shufflemask);
                          ^
test/utils-prng.c:207:25: error: assigning to 'uint8x16' (vector of 16 'uint8_t' values) from incompatible type 'int'
            randdata.vb = __builtin_shuffle (randdata.vb, bswap_shufflemask);
                        ^ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
2 errors generated

Link to original discussion:
http://lists.llvm.org/pipermail/cfe-dev/2017-August/055140.html

It's possible to build pixman if attached patch is applied. Basically
patch adds check for __builtin_shuffle support and in case there is
none, falls back to clang-specific __builtin_shufflevector that do the
same but have different API.

Bugzilla: https://bugs.gentoo.org/646360
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=104886


Tested-by: Philip Chimento <philip.chimento@gmail.com>
Reviewed-by: Matt Turner <mattst88@gmail.com>
Reviewed-by: Adam Jackson <ajax@redhat.com>