benchmarks: Record and replay calls to EXECBUFFER2

This slightly idealises the behaviour of clients with the aim of
measuring the kernel overhead of different workloads. This test focuses
on the cost of relocating batchbuffers.

A trace file is generated with an LD_PRELOAD intercept around
execbuffer, which we can then replay at our leisure. The replay replaces
the real buffers with a set of empty ones so the only thing that the
kernel has to do is parse the relocations. but without a real workload
we lose the impact of having to rewrite active buffers.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>

benchmarks/.gitignore

@@ -1,6 +1,7 @@
 gem_create
 gem_exec_ctx
 gem_exec_nop
+gem_exec_trace
 gem_mmap
 gem_prw
 gem_userptr_benchmark