[OpenMP][SystemZ] Compile __kmpc_omp_task_begin_if0() with backchain (llvm#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.

Author: iii-i

openmp/runtime/src/kmp_tasking.cpp

@@ -839,6 +839,14 @@ static void __kmpc_omp_task_begin_if0_ompt(ident_t *loc_ref, kmp_int32 gtid,
 // loc_ref: source location information; points to beginning of task block.
 // gtid: global thread number.
 // task: task thunk for the started task.
+#ifdef __s390x__
+// This is required for OMPT_GET_FRAME_ADDRESS(1) to compile on s390x.
+// In order for it to work correctly, the caller also needs to be compiled with
+// backchain. If a caller is compiled without backchain,
+// OMPT_GET_FRAME_ADDRESS(1) will produce an incorrect value, but will not
+// crash.
+__attribute__((target("backchain")))
+#endif
 void __kmpc_omp_task_begin_if0(ident_t *loc_ref, kmp_int32 gtid,
                                kmp_task_t *task) {
 #if OMPT_SUPPORT