graphics/R-cran-s2: fix build on powerpc64

c++ -std=gnu++11 -I"/usr/local/lib/R/include" -DNDEBUG -I../inst/include -DSTRICT_R_HEADERS -I'/usr/local/lib/R/library/Rcpp/include' -I'/usr/local/lib/R/library/wk/include' -DLIBICONV_PLUG -I/usr/local/include -isystem /usr/local/include -DIS_BIG_ENDIAN -pthread -fPIC -O2 -pipe -DLIBICONV_PLUG -fstack-protector-strong -isystem /usr/local/include -fno-strict-aliasing -DLIBICONV_PLUG -isystem /usr/local/include -c s2-accessors.cpp -o s2-accessors.o
In file included from s2-accessors.cpp:2:
In file included from ./geography-operator.h:7:
In file included from ./geography.h:8:
In file included from ../inst/include/s2/s2polygon.h:29:
In file included from ../inst/include/s2/mutable_s2shape_index.h:34:
In file included from ../inst/include/s2/s2cell_id.h:36:
../inst/include/s2/util/bits/bits.h:509:19: error: this builtin is only valid on POWER7 or later CPUs

uint64 result = __builtin_bpermd(0x3f3e3d3c3b3a3938, temp);