[libc] Declare __dso_handle in the integration test instead of startup.

Fixes #61355. The __dso_handle decl was introduced incorrectly into the startup
objects during the integration test cleanup which moved the integration tests
away from using an artificial sysroot to using -nostdlib. Having it in the
startup creates the duplicate symbol error when one does not use -nostdlib.
Since this is an integration test only problem, it is meaningful to keep it in
the integration test anyway.

Differential Revision: https://reviews.llvm.org/D145898