Downgrade optimization level from -O3 to -O2.

Summary:
When we use -O3, the gcc 4.7.1 compiler generates 'pinsrd' which is
not supported on machines with "vendor_id       : AuthenticAMD".

Previous release of rocksdb used -O2.
Optimization -O2 was introduced at
772f75b3fb

Test Plan: make check

Reviewers: chip, heyongqiang, sheki

Reviewed By: sheki

CC: leveldb

Differential Revision: https://reviews.facebook.net/D9093

Makefile

@@ -10,7 +10,7 @@ INSTALL_PATH ?= $(CURDIR)
 # to switch between compilation modes.
 
 # OPT ?= -DNDEBUG     # (A) Production use (optimized mode)
-OPT += -O3 -fno-omit-frame-pointer -momit-leaf-frame-pointer
+OPT += -O2 -fno-omit-frame-pointer -momit-leaf-frame-pointer
 #-----------------------------------------------
 
 # detect what platform we're building on