mem: implement ThreadBridge

ThreadBridge is used for communication between two SimObjects from
different threads (EventQueue).

Change-Id: I3e00df9184404599dfacef64b505cd0b64ee46aa
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/65071
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Reviewed-by: Yu-hsin Wang <yuhsingw@google.com>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>
4 files changed