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 b5c8a3aa82
("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.

sysdeps/unix/sysv/linux/nios2/localplt.data

@@ -26,6 +26,7 @@ libc.so: __nedf2
 libc.so: __eqdf2
 libc.so: __extendsfdf2
 libc.so: __floatundidf ?
+libc.so: __floatunsidf ?
 libm.so: matherr
 # The TLS-enabled version of these functions is interposed from libc.so.
 ld.so: _dl_signal_error