This patch adds support for testing _Float64x libm functions. A
configuration with such functions sets float64x-alias-fcts = yes in a
sysdeps Makeconfig file; until such settings are added, this test
support is inactive.
Tested for x86_64, including in conjunction with _Float64x support
patches.
* math/test-float64x.h: New file.
* math/Makefile (type-float64x-yes): New variable.
(test-types): Add $(type-float64x-$(float64x-alias-fcts)).
