[trace][intel pt] Support a new kernel section in LLDB’s trace bundle schema

Add a new "kernel" section with following schema.

```
"kernel": {
  "loadAddress"?: decimal | hex string | string decimal
  # This is optional. If it's not specified, use default address 0xffffffff81000000.
  "file": string
  # path to the kernel image
}
```

Here's more details of the diff:
- If "kernel" section exist, it means current tracing mode is //KernelMode//.
- If tracing mode is //KernelMode//, the "processes" section must be empty and the "kernel" and "cpus" section must be provided. This is tested with `TestTraceLoad`.
- "kernel" section is parsed and turned into a new process with a single module which is the kernel image. The kernel process has N fake threads, one for each cpu.

Reviewed By: wallace

Differential Revision: https://reviews.llvm.org/D130805