Update README.
Update throughput figure.

Signed-off-by: Xiaofeng Lin <jerrylin.lxf@gmail.com>
1. Export the CPP_VERS to fix the compatibility issue of gcc;
2. Use -m64 and -m32 instead of the -m elf_x86_64 and -m elf_i386 separately
   when the gcc version is higher than 4.6.
   Because gcc does not support the linker-style since 4.6