graphics/mesa-libs: fix build on powerpc64le

Checking if "POWER8 intrinsics" compiles: NO

meson.build:720:6: ERROR: Problem encountered: POWER8 intrinsic support required but not found.

graphics/mesa-dri/files/patch-meson.build

+--- meson.build.orig	2023-09-20 17:15:34 UTC
++++ meson.build
+@@ -709,7 +709,7 @@ if _power8.allowed()
+         int main() {
+           vector unsigned char r;
+           vector unsigned int v = vec_splat_u32 (1);
+-          r = __builtin_vec_vgbbd ((vector unsigned char) v);
++          r = __builtin_altivec_vgbbd ((vector unsigned char) v);
+           return 0;
+         }''',
+         args : '-mpower8-vector',