QOL improvements for hang-analyzer

1) Log a raw_stacks file for each process instead of one file for all processes (this was the behavior prior to SERVER-46682). The 'debugger_mongod.XXXXX.log' files that we used to get are now all in the 'debugger_mongod.log' file, but we can't split those out into per process logs because we only call gdb once.

2) Pass '-o=file' into the hang-analyzer usage in assert.soon to get the hang-analyzer output as an artifact in the task.