[libc] Add support for global ctors / dtors for AMDGPU

This patch makes the necessary changes to support calling global
constructors and destructors on the GPU. The patch in D149340 allows the
`lld` linker to create the symbols pointing us to these globals. These
should be executed by a single thread, which is more difficult on the
GPU because all threads are active. I chose to use an atomic counter to
sync every thread on the GPU. This is very slow if you use more than a
few thousand threads, but for testing purposes it should be sufficient.

Depends on D149340 D149363

Reviewed By: sivachandra

Differential Revision: https://reviews.llvm.org/D149398