math tests on amd64

[Greg Troxel]

I ran 'make check' on guile on NetBSD-current on amd64, and got a
failure:

guile> (version)
"1.8.5"
guile> (eqv? 0.0 (exp -inf.0))
#f
guile> (exp -inf.0)
+nan.0

On i386 and sparc64 this passes.  I ran paranoia and that passed (with
not even flaws).  Does this test pass for others on other operating
systems on amd64?  If so, I should look into whether it's a NetBSD bug,
variance in floating point flags, or a guile bug.

So far this looks like a failure of NetBSD on amd64 to follow POSIX:
http://www.opengroup.org/onlinepubs/009695399/functions/exp.html