[DirectVerbs] Make the code compilable without exp-verbs

[pakmarkthub]

Prerequisite:

• PR [DirectVerbs] Remove all exp-verbs usage from all files except from mlx5-exp.* #102.

Problem:

• Until PR [DirectVerbs] Remove all exp-verbs usage from all files except from mlx5-exp.* #102, other parts of libgdsync call functions in mlx5-exp.cpp directly. As mlx5-exp requires exp-verbs headers, we are not completely separate from exp-verbs.

This PR:

• defines a transport interface.
• modifies mlx5-exp.cpp to register all functions through this interface during the initialization phase.
• edits the libgdsync code that calls functions in mlx5-exp.cpp to call them via the transport interface.
• modifies configure.ac to compile mlx5-exp.cpp when exp-verbs is detected only.
• fixes bugs when compiling on x86.

Presubmit testing:

• compiled and ran on Summit.
• compiled on gc11.