Add strfromd, strfromf, and strfroml functions

ISO/IEC TS 18661-1 adds several functions in the strfrom family to stdlib.
This patch adds strfromd, strfromf, and strfroml.  This is being done in
preparation for the new floating-point type, float128.  The added functions
convert a floating-point value into a string, with configurable format.