double test failures on AMD machines

[Eric Wong]

Hey, I'm hitting some test failures on current master[1] on
two of my AMD x86_64 machines, logs + /proc/cpuinfo here:

http://yhbt.net/w/test-ildoubl.out.gz
http://yhbt.net/w/test-ldouble.out.gz
http://yhbt.net/w/cpuinfo.gz

http://yhbt.net/s/test-ildoubl.out.gz
http://yhbt.net/s/test-ldouble.out.gz
http://yhbt.net/s/cpuinfo.gz

Both are Debian 7.0 x86_64 systems, built with
gcc (Debian 4.7.2-5) 4.7.2
Nothing non-default in ./configure besides --prefix=/tmp/g
Copied libgcc_s.so.1 and libstdc++.so.6 over from the OS installation
over to /tmp/g/lib

I couldn't reproduce these on an Intel Haswell machine.
Was testing for [BZ #15132] avoid stat/fstat in statvfs/fstatvfs,
but it happens without my patch (and my patch doesn't touch FP :)

[1] commit 409e00bd69b8d8dd74d7327085351d26769ea6fc