[llvm-exegesis] Make sure auxiliary memory is unlinked after usage

Currently the shm_unlink is never called on the auxiliary memory shared
memory, so it stays around after running llvm-exegesis (including
running the llvm-exegesis tests). This patch adds the auxiliary memory
shared memory name to the SharedMemoryNames list so that it gets
unlinked when the destructor of SubprocessMemory is called.