Summary:
This diff changes compile to optimize for native platform by default. This will automatically turn on crc32 optimizations for modern processors, which greatly improves rocksdb's performance.
I also did some more changes to compilation documentation.
Test Plan:
compile with `make`, observe -march=native
compile with `PORTABLE=1 make`, observe no -march=native
Reviewers: sdong, rven, yhchiang, MarkCallaghan
Reviewed By: MarkCallaghan
Subscribers: dhruba, leveldb
Differential Revision: https://reviews.facebook.net/D30225