Allow build_all_ios.sh to build just one arch (#13959)

This change allows build_all_ios.sh to take an -a flag with a
specific arch so you dont have to waste time building unwanted
architectures (32bit etc)

TEST: tensorflow/contrib/makefile/build_all_ios.sh  #builds fat lib
tensorflow/contrib/makefile/build_all_ios.sh -a "arm64" #only arm64

An incremental build with just touching one file with

time ./tensorflow/contrib/makefile/build_all_ios.sh -T -a arm64

real    0m8.845s
user    0m6.934s
sys     0m1.638s