build: revisit x86_64 build flags yet again

Motivation to add -msse conditionally:

On x86_64, the kernel specifies -mno-sse, with the intention of detecting
accidental use of FP in kernel space.

gcc -m64 -mno-sse rejects the following code, because the ABI places the
return value of double sin(double) in the SSE register %xmm0 (and/or
because the argument :
    extern double sin(double);
    double d;
    void fn() {
        d = sin(d);
    }

rtai for x86_64 is supposed to put -msse in $(rtai-config
--module-cflags), but in rtai-modules-2.6.24-16-rtai 3.6.1-linuxcnc.2
(ubuntu hardy amd64) this is not the case.

Detect this and add -msse by hand if it is not there.  Also, take care
to add it only in kernel space; we're not aware of any reason to have to
add it in userspace.

Motivation to remove -ffast-math, -f[no-]unsafe-math-optimizations:

gcc 4.7.2-5 on debian wheezy miscompiles
    double val;

    void fn(double dot, double radius) {
        val = dot / (radius * radius);
    }
under -Os -ffast-math.  -ffast-math implies -funsafe-math-optimizations,
so specifying it was redundant.  In 2011 I thought I had a report that
-ffast-math -funsafe-math-optimizations was required on one user's amd64
setup, but now that same user reports he has to drop those flags to get
correct behavor, so as far as I know we have no users who need the
-ffast-math flag. (this bug is upstream and fixed:
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=56125 but the fix looks
unlikely to make it into Debian 7.0's initial release)
Revert "fix building without 5axiskins"

This reverts commit d2bdfa68cfa0206f558db61d9c97f30e9c5483e3.
Revert "Remove bogus sample kins file"

This reverts commit 62d11480ab6091e7d5676884e4d5267218eccf07.
The 5axis sample config uses this!
The tool-length hal pin was vestigial and confusing.