[OpenMP][Offloading] Change `bool IsSPMD` to `int8_t Mode` in...

[OpenMP][Offloading] Change `bool IsSPMD` to `int8_t Mode` in `__kmpc_target_init` and `__kmpc_target_deinit`

This is a follow-up of D110029, which uses bitset to indicate execution mode. This patches makes the changes in the function call.

Reviewed By: jdoerfert

Differential Revision: https://reviews.llvm.org/D110279