We want to be backtracing directly into the capture buffer, but also need
to skip a small number of frames.
If we call the backtrace before filling in information, we can capture to
the position *before* ev->addrs and then overwrite that data right after.
This frame type can be used to communicate with the peer over the mapped
ring buffer to denote that writing is finished and it can free any
resources for the mapping.
This is a simplified API for the inferior to use (such as from a
LD_PRELOAD) that will use mmap()'d ring buffer created by Sysprof. Doing
so can reduce the amount of overhead in the inferior enough to make some
workloads useful. For example, collecting memory statistics and backtraces
is now fast enough to be useful.
