traced_perf: allow collection and symbolization of kernel frames

If a config asks for kernel frames, we include PERF_SAMPLE_CALLCHAIN in
the event config. The kernel unwinds its own frames, and we only need to
symbolize them in the unwinder, using the kallsyms symbolizer that was
added for traced_probes.

The kernel frames are output as part of the normal callstack, and can
be further identified by having a magical "kernel" string for their
mapping name. This is similar to what we do for synthetic error frames.

Tested with aosp/1496216 for the SELinux changes around /proc/kallsyms
access on android.

Bug: 173124818
Change-Id: I05ef78621d00c60e05bb4833c63505c910a0928a