[OpenMP][deviceRTLs] Fix wrong return value of `__kmpc_is_spmd_exec_mode`

D110279 introduced a bug to the device runtime. In `__kmpc_parallel_51`, we detect
whether we are already in parallel region by `__kmpc_parallel_level() > __kmpc_is_spmd_exec_mode()`.
It is based on the assumption that:
- In SPMD mode, parallel level is initialized to 1.
- In generic mode, parallel level is initialized to 0.
- `__kmpc_is_spmd_exec_mode` returns `1` for SPMD mode, 0 otherwise.

Because the return value type of `__kmpc_is_spmd_exec_mode` is `int8_t`, there
was an implicit cast from `bool` to `int8_t`. We can make sure it is either 0 or
1 since C++14. In D110279, the return value is the result of an `and` operation,
which is 2 in SPMD mode. This breaks the assumption in `__kmpc_parallel_51`.

Reviewed By: carlo.bertolli, dpalermo

Differential Revision: https://reviews.llvm.org/D111905