[flang][OpenMP] Fix FlangRuntime offload build after assign change

[dpalermo]

- Use actual function for memmove (address taken)

[dpalermo]

Fixed needed after change in llvm#114301

[dpalermo]

Also interesting, but the pre-existing libc replacement memmove was missing a return value on the early exit case. Now fixed. Reported as a compilation error after no longer using the memmove builtin.

[dpalermo]

Raghu noticed the GenASiS target test was no longer finding -lFortranRuntimeHostDevice ...as the build of libFortranRuntimeHostDevice.a was failing.

[raghavendhra]

Thanks for fixing this :)