math/asl: fix build on powerpc*

Don't use fpsetprec / fpsetmask on powerpc*:
fpinit.c:246:2: warning: implicit declaration of function 'fpsetprec' is invalid in C99 [-Wimplicit-function-declaration]

fpsetprec(FP_PD);
^

fpinit.c:246:12: error: use of undeclared identifier 'FP_PD'

fpsetprec(FP_PD);