Support defining strfromf64, strfromf32x aliases.
This patch adds support for defining strfromf64 and strfromf32x
aliases of strfromd when the corresponding types are enabled.
Tested for x86_64; also tested with build-many-glibcs.py in
conjunction with other _Float64 / _Float32x changes.
* stdlib/strfromd.c: Include <bits/floatn.h>.
[__HAVE_FLOAT64 && !__HAVE_DISTINCT_FLOAT64] (strfromf64): Define
and later undefine as macro and define as weak alias.
[__HAVE_FLOAT32X && !__HAVE_DISTINCT_FLOAT32X] (strfromf32x):
Likewise.