[vm] Rearrange some members of Thread to keep offsets within RISC-V I-type range.

TEST=ci
Cq-Include-Trybots: luci.dart.try:vm-linux-debug-simriscv32-try,vm-linux-debug-simriscv64-try,vm-aot-linux-debug-simriscv32-try,vm-aot-linux-debug-simriscv64-try
Change-Id: Ifbdb22a4454b7a6fefa44a62e4f69ea36adb1e1d
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/442563
Commit-Queue: Ryan Macnak <[email protected]>
Reviewed-by: Alexander Markov <[email protected]>

Author: rmacnak-google