This patch adds the headers required for testing _Float64 and
_Float32x function aliases (using double ulps). The corresponding
makefile support will be included in the patch that actually adds
those aliases; there doesn't seem much point in adding makefile
conditionals for testing something that will be available
unconditionally.
In conjunction with other _Float64 / _Float32x changes, test for
x86_64 and with build-many-glibcs.py.
* math/test-float32x.h: New file.
* math/test-float64.h: Likewise.
