new test: return small struct

new test: return small struct

The bug originally was discovered in https://bugs.gentoo.org/634190
where complicated callback was returning invalid data on ia64.

This change adds minimal reproducer that fails only on ia64 as:

FAIL: libffi.call/struct10.c -W -Wall -Wno-psabi -O0 execution test
FAIL: libffi.call/struct10.c -W -Wall -Wno-psabi -O2 execution test
FAIL: libffi.call/struct10.c -W -Wall -Wno-psabi -O3 execution test
FAIL: libffi.call/struct10.c -W -Wall -Wno-psabi -Os execution test

Test passes on amd64. The fix is in the following commit.

Bug: https://bugs.gentoo.org/634190
Signed-off-by: Sergei Trofimovich <slyfox@gentoo.org>