On ports without __HAVE_LONG_DOUBLE, make fabsl alias fabs.

For some reason, fabs lives in libc, not in libm, and our tests now
detect when fabs or fabsl is missing from libm.  For those ports that
sometimes have long double and sometimes don't, make it conditional.

Still missing: fabs _and_ fabsl on ia64.  Need help from an itanium
wizard!  Other portmasters: Please take a look and see if I missed
any ports that might have long double where this alias will not work.

(riastradh)