Implement fenv for more architectures

[Martin Husemann <martin%duskware.de@localhost>]

Hey folks,

more and more parts of our source tree depend on fenv.h (of course mostly
in libm). Fenv(3) handles things like FPU rounding mode and exception 
handling.

It is currently available for amd64, arm, i386, sparc and sparc64.

It should be pretty easy to implement for other architectures, and it would
be great to complete support across all arches, to avoid ugly ifdefs like
I just hacked into libm/src/e_sqrtl.c.

Any volunteers?

Martin