build system: consolidate duplicate code and more

* add ability to add assembly files when building an exe, obj, or lib
 * add implicit cast from `[N]T` to `?[]const T` (closes #343)
 * remove link_exe and link_lib in favor of allowing build_exe and
   build_lib support no root zig source file
7b0542d08bAndrew Kelley committed on 4/26/2017, 11:17:05 PMparent09bc4d6
13 files changedLine totals unavailable