在用file查看library的architechture时有以下输出：
$ file WebP
WebP: Mach-O universal binary with 3 architectures
WebP (for architecture i386): current ar archive random library
WebP (for architecture armv7): current ar archive random library
WebP (for architecture cputype (12) cpusubtype (11)): current ar archive random library
architecture cputype (12) cpusubtype (11) 标识的硬件架构是armv7s。
http://stackoverflow.com/questions/12549489/compile-library-for-armv7s-cputype-12-and-cpusubtype-11