[OpenMP][SystemZ] Compile __kmpc_omp_task_begin_if0() with backchain (#71834)

OpenMP runtime fails to build on SystemZ with clang with the following
error message:

    LLVM ERROR: Unsupported stack frame traversal count

__kmpc_omp_task_begin_if0() uses OMPT_GET_FRAME_ADDRESS(1), which
delegates to __builtin_frame_address(), which in turn works with nonzero
values on SystemZ only if backchain is in use. If backchain is not in
use, the above error is emitted.

Compile __kmpc_omp_task_begin_if0() with backchain. Note that this only
resolves the build error. If at runtime its caller is compiled without
backchain, __builtin_frame_address() will produce an incorrect value,
but will not cause a crash. Since the value is relevant only for OMPT,
this is acceptable.