rtapi_string: actually export these APIs

move backported-strings to uspace_rtapi_string.c.  These are unneeded
in kernelspace since kernel 2.6.23, and our minimum required kernel is
2.6.32, so we only need to provide an implementation in userspace.
hm2: drop unneeded workaround for old kernels
rtapi_math64: drop unneeded workaround for old kernels
rtai: drop unneeded workaround for old kernels