Make register_target methods thread-safe

[bpickrel]

Make methods in register_target.cpp thread-safe and create a test for handling of register actions concurrent threads.

The method migraphx::version_2_6_0::make_target() was flagged by valgrind as causing a thread safety error. This in turn caused one of the UIF Inference Server examples, example_resnet50-migraphx, to experience a SIGSEGV error.

This error was first seen when UIF bumped ROCm versions from 5.4.x to 5.6.1.

[bpickrel]

Created draft pull request 2224

[bpickrel]

Placing mutexes on various methods in register_target.cpp did not prevent crashes. Segment violations occurred most often in the implicit destructor of class dynamic_loader, so the problem may lie in this class instead of the register_target methods.

[bpickrel]

Built amdinfer with the contents of PR #2224 and it still encounters the same error. Not closed.