TS 18661-1 adds an issubnormal classification macro to <math.h>. This
patch implements it for glibc. There are no new underlying functions
in libm because the implementation uses fpclassify; any optimizations
for this macro should be done through adding __builtin_subnormal in
GCC and using it in the header for suitable GCC versions, not through
adding other optimized inline or out-of-line versions to glibc.
The intended structure of the NEWS entry for <math.h> features from TS
18661-1 is like:
* New <math.h> features are added from TS 18661-1:2014: