graphics/s2: fix build on powerpc64

Specifically optimize for POWER7 or later:
/wrkdirs/usr/ports/graphics/s2/work/s2geometry-0.9.0/src/s2/util/bits/bits.h:509:19: error: this builtin is only valid on POWER7 or later CPUs

uint64 result = __builtin_bpermd(0x3f3e3d3c3b3a3938, temp);