* CLI: `-target [name]` instead of `--target-*` args.
This matches clang's API.
* `builtin.Environ` renamed to `builtin.Abi`
- likewise `builtin.environ` renamed to `builtin.abi`
* stop hiding the concept of sub-arch. closes#1526
* `zig targets` only shows available targets. closes#438
* include all targets in readme, even those that don't
print with `zig targets` but note they are Tier 4
* refactor target.cpp and make the naming conventions
more consistent
* introduce the concept of a "default C ABI" for a given
OS/Arch combo. As a rule of thumb, if the system compiler
is clang or gcc then the default C ABI is the gnu ABI.
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