gdb/testsuite/gdb.rocm: Check value returned by hipDeviceSynchronize
Functions of the hip runtime returning a hipError_t can be marked
nodiscard depending on the configuration[1] (when compiled with C++17).
This patch makes sure that we always check the value returned by
hipDeviceSynchronize and friends, and print an error message when
appropriate. This avoid a wall of warnings when running the testsuite
if the compiler defaults to using C++17.
It is always a good practice to check the return values anyway.