librustc: ar call fix to support android cross compile on mac
librustc: move target dependent logic to back::link
librustc: add get_system_tools for target specific environment
librustc: tidy cleanup
auto merge of #10900 : yichoi/rust/mac_android_cross, r=alexcrichton

this patch should be followed by https://github.com/alexcrichton/libuv/pull/2