Allow memset local PLT reference for powerpc soft-float.

Some recent change on GCC mainline resulted in the localplt test
failing for powerpc soft-float (not sure exactly when, as the failure
appeared when there were other build test failures as well;
<https://sourceware.org/ml/libc-testresults/2019-q2/msg00261.html>
shows it remaining when other failures went away).  The problem is a
call to memset that GCC now generates in the libgcc long double code.

Since memset is documented as a function GCC may always implicitly
generate calls to, it seems reasonable to allow that local PLT
reference (just like those for libgcc functions that GCC implicitly
generates calls to and that are also exported from libc.so), which
this patch does.

Tested for powerpc soft-float with build-many-glibcs.py.

	* sysdeps/unix/sysv/linux/powerpc/powerpc32/nofpu/localplt.data:
	Allow memset in libc.so.