benchmarks: Benchmarkify gem_exec_ctx

Measure the overhead of execution when doing nothing, switching between
a pair of contexts, or creating a new context every time.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
diff --git a/benchmarks/.gitignore b/benchmarks/.gitignore
index 68f8f1d..7964d86 100644
--- a/benchmarks/.gitignore
+++ b/benchmarks/.gitignore
@@ -1,3 +1,4 @@
+gem_exec_ctx
 gem_exec_nop
 gem_mmap
 gem_prw