math/oink: fix build on powerpc*

/wrkdirs/usr/ports/math/oink/work/oink-c1259fe/src/lace.c:973:9: error: invalid instruction

mfence();
^

/wrkdirs/usr/ports/math/oink/work/oink-c1259fe/src/lace.h:296:33: note: expanded from macro 'mfence'

^

<inline asm>:1:2: note: instantiated into assembly here

mfence