Run $(objpfx)iconvconfig with $(run-program-prefix) [BZ #27477]

When glibc is configured with --enable-hardcoded-path-in-tests,
"make xcheck" failed with

...
env GCONV_PATH=/export/build/gnu/tools-build/glibc-cet-gitlab/build-x86_64-linux/iconvdata LOCPATH=/export/build/gnu/tools-build/glibc-cet-gitlab/build-x86_64-linux/localedata LC_ALL=C  /export/build/gnu/tools-build/glibc-cet-gitlab/build-x86_64-linux/iconv/iconvconfig  --output=$tmp --nostdlib /usr/lib64/gconv;
...
/export/build/gnu/tools-build/glibc-cet-gitlab/build-x86_64-linux/iconv/iconvconfig: /lib64/libc.so.6: version `GLIBC_2.34' not found (required by /export/build/gnu/tools-build/glibc-cet-gitlab/build-x86_64-linux/iconv/iconvconfig)
...
FAIL: iconv/test-iconvconfig

Since $(objpfx)iconvconfig is an installed program, run it with
$(run-program-prefix).