[PPC64] g1_write_barrier_pre needs extension for Loom

Loom requires the c2i_entry_barrier which is currently not supported with G1 on PPC64. (nmethod entry barriers were not needed for G1 before.)

We run into the following assertion when trying to start the VM with Loom enabled:
assert(preservation_level <= MacroAssembler::PRESERVATION_FRAME_LR) failed: g1_write_barrier_pre doesn't support preservation levels higher than PRESERVATION_FRAME_LR

The following frames are on stack:
G1BarrierSetAssembler::g1_write_barrier_pre
G1BarrierSetAssembler::load_at
MacroAssembler::resolve_weak_handle
BarrierSetAssembler::c2i_entry_barrier