isfinite, isinf, isnan tests: fix for little-endian PowerPC * tests/test-isfinite.c (test_isfinitel): Only manipulate the first double of a PowerPC "double double" pair. * tests/test-isinf.c (test_isinfl): Likewise. * tests/test-isnan.c (test_long_double): Likewise. * tests/test-isnanl.h (main): Likewise. * tests/test-signbit.c (test_signbitl): Likewise.