elf: Fix tests that rely on ld.so.cache for cross-compiling

For configurations with cross-compiling equal to 'maybe' or 'no',
ldconfig will not run and thus the ld.so.cache will not be created
on the container testroot.pristine.

This lead to failures on both tst-glibc-hwcaps-prepend-cache and
tst-ldconfig-ld_so_conf-update on environments where the same
compiler can be used to build different ABIs (powerpc and x86 for
instance).

This patch addas a new test-container hook, ldconfig.run, that
triggers a ldconfig execution prior the test execution.

Checked on x86_64-linux-gnu and i686-linux-gnu.