[libc] Actually run integration tests.

After the switch to `add_custom_target` to run integration tests, most
of them were not actually being run because of the difference in the way
the COMMAND value is treated between `add_custom_target` and
`add_custom_command`. This patch gets the integration tests to run
again by passing the correct set of arguments to `add_custom_target`.

Reviewed By: michaelrj

Differential Revision: https://reviews.llvm.org/D148786