[libc] Add a simple implementation of the posix_spawn function.

The implementation currently ignores all spawn attributes. Support for
them will be added in future changes.

A simple allocator for integration tests has been added so that the
integration test for posix_spawn can use the
posix_spawn_file_actions_add* functions.

Reviewed By: michaelrj

Differential Revision: https://reviews.llvm.org/D135752