change, we should be fully conformant with the N32 and N64 ABIs.
Add {fpclassify,infinity,isnan,ininf,signbit}l_ieee754.c back to lib/libc/gen.
Note that infinityl_ieee754.c will work with either 64-bit, 80-bit, or
128-bit long doubles.
keep a common implementation of isinfl() and isnanl() to be used by
platforms where `long double' == `double'; move others into
machine-dependent code.
* In due course, consider __VFP_FP__ on arm.