iconvdata/run-iconv-test.sh: Actually test iconv modules

Arjun Shankar noticed that this test case was not testing anything
because iconv was invoked without the required arguments.
* string/stpcpy.c (__stpcpy): Rewrite using strlen and memcpy.
tilegx: enable wordsize-64 support for ieee745 dbl-64.

I missed this during the initial port.  Some testing shows that
enabling this mode does, unsurprisingly, yield some nice speedups
on the math functions in question.