gdb/testsuite/gdb.rocm: Check value returned by hipDeviceSynchronize

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.

[1] https://github.com/ROCm-Developer-Tools/HIP/blob/docs/5.7.1/include/hip/hip_runtime_api.h#L203-L218

Change-Id: I2a819a8ac45f4bcf814efe9a2ff12c6a7ad22f97
Approved-By: Simon Marchi <simon.marchi@efficios.com>