Support _Float32 in libm_alias_float.
This patch makes the libm_alias_float macro support creating _Float32
aliases, in preparation for enabling glibc support for that type.
Tested for x86_64; also tested with build-many-glibcs.py in
conjunction with other _Float32 changes.
* sysdeps/generic/libm-alias-float.h: Include <bits/floatn.h>.
[__HAVE_FLOAT32 && !__HAVE_DISTINCT_FLOAT32]
(libm_alias_float_other_r): Create f32 alias.
(libm_alias_float_r): Use semicolon before call to
libm_alias_float_other_r.