testsuite: fix shared library size tests for cross ghc

This commit fixes shared library size tests (e.g. array_so in
testsuite/tests/perf/size/all.T) when testing cross ghc. Previously,
if shared library file extension of host and target differs, those
tests will fail with framework errors due to not finding the right
files.

(cherry picked from commit 05e40406)
(cherry picked from commit d00c26a5)
(cherry picked from commit 908d6621)