libbacktrace: let the benchmark library decide iteration count.

Manually doing 1000 iterations of the benchmark doesn't seem to add any
significant amount of precision, and it makes the benchmark take
forever and obfuscates the results. Just let benchmark figure out the
time (with the option of using command line flags to increase the
number of iterations).

Test: backtrace_benchmarks64
Change-Id: I8de912c1b3c904755c8e2ac4175ff70176544ba3