The implementation of run-iconv-test.sh will run icong_prov specifying a
GCONV_PATH for the current glibc build. Then run the cmp (from the base
install) to check the results.

This can be a problem because currently cmp will run with the same
GCONV_PATH as iconv_prog and cmp does try to load ISO8859-1.so. Since
GCONV_PATH is pointing into the current build it will try to load from there.

This really gets sticky when there is a ABI or symbol version difference
between the glibc build and the base install. For example if there are
new relocations which ld.so from the base install does not understand
(i.e. the new relocations associated with -msecure-plt). This will cause
cmp to fail (segfault in the loader associated with cmp) and all the
iconv-tests to fail.

The attached patch resolves this by temporarily resetting GCONV_PATH for
cmp commands.