Only test ld_pi_odd with LDBL_MANT_DIG == 64 to fix the build

The empty (unimplemented) test inputs for sparc64 trigger a -Wtype-limits build
failure because nitems of an empty array is always false, i.e., deadcode.

MFC after: 1 month
MFC with: r321455
Reported by: Jenkins (sparc64 job)