science/cgribex: fix build on powerpc64le, powerpc64 with CPUTYPE=power6 and newer with Clang

Clang doesn't have __fsel function.
Taken from https://gcc.gnu.org/git/?p=gcc.git;a=blob;f=gcc/config/rs6000/ppu_intrinsics.h;h=e17faae05932e33c22697a3178b26314b88c9a31;hb=HEAD#l530