traced_probes: function_graph recording

Add ability to configure "function_graph" as the active ftrace tracer,
which will emit funcgraph_entry and funcgraph_exit events into the
ringbuffer (alongside whatever other tracepoints have been enabled).
Parse these events, using the kallsyms symboliser to resolve the
function pointers to names. Also add support for specifying which
functions to trace using the "set_ftrace_filter" interface.

Notable limitation: if there is already a "nop" tracer ftrace data
source active, the function graph data source will be rejected since the
kernel will refuse (EBUSY) to change the active tracer if the trace
pipes are already opened (and I'm not willing to refactor traced_probes
to support closing and reopening pipes during an active trace). However
the opposite order (starting a tracepoint-only trace after a funcgraph
one) is ok. Concurrent funcgraph data sources are supported, but they
will all observe the full set of traced functions.

Android: requires a debug build for now.

Based on a contribution from bgrzesik@.

Bug: 249767431
Change-Id: Ic045a26f89ce4b24559d090c68d7c6c7a132c4c3