logwrapper: add a benchmark for android_fork_execvp_ext

Current results on an aosp_angler (32 bit) :

Benchmark                           Time           CPU Iterations
-----------------------------------------------------------------
BM_android_fork_execvp_ext   15019207 ns    1002131 ns        700

Test: android_fork_execvp_ext_benchmark

Change-Id: I3d4339ff5cbb9a641cbb14f0b55d2b5fc18cdfde