Source file structure for fenv.h support

[Joel Sherrill]

Hi

Vaibhav is looking at porting *BSD fenv support for a few more
architectures. Right now, it looks like there is only SPU and RISC-V
support with this structure:

- libc/machine/ARCH/include/fenv.h - portable POSIX interface, includes
sys/fenv.h
- libc/machine/ARCH/include/sys/fenv.h - architecture specific details.

I think (but have not confirmed) that the RISC-V version is correct per
POSIX. The SPU version clearly is not as it does not have the right
signature for most of the methods as they all are supposed to return int
and not void.

My proposal is to verify the RISC-V include/fenv.h is POSIX correct and
then move it to libc/include/fenv.h.For completeness, we may need a
minimal, non-functional sys/fenv.h which defines dummy fexcept_t and fenv_t
structures.  But it becomes the responsibility of a port to provide
<sys/fenv.h> and implementations.

In the event a port provides the entire implementation as inlines, it can
override fenv.h.

Does this make sense? Is it a good approach? I want to avoid duplicating a
pure POSIX version of fenv.h like RISC-V has.

Thanks.

--joel