Fix nios2 localplt failure

Building for nios2-linux-gnu has recently started showing a localplt
test failure, arising from a reference to __floatunsidf from
getloadavg after commit b5c8a3aa
("Linux: implement getloadavg(3) using sysinfo(2)") (this is an
architecture with soft-fp in libc).  Add this as a permitted local PLT
reference in localplt.data.

Tested with build-many-glibcs.py for nios2-linux-gnu.