[libFuzzer] Let user specify extra stats file.

Summary: If AFL_DRIVER_EXTRA_STATS_FILENAME is set and valid, write to it peak_rss_mb and slowest_unit_time_sec. These are both stats that libFuzzer can print but afl cannot.

Reviewers: kcc, aizatsky, metzman

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D21742

llvm-svn: 274273