* re-introduce `std.build.Target` which is distinct from `std.Target`.
`std.build.Target` wraps `std.Target` so that it can be annotated as
"the native target" or an explicitly specified target.
* `std.Target.Os` is moved to `std.Target.Os.Tag`. The former is now a
struct which has the tag as well as version range information.
* `std.elf` gains some more ELF header constants.
* `std.Target.parse` gains the ability to parse operating system
version ranges as well as glibc version.
* Added `std.Target.isGnuLibC()`.
* self-hosted dynamic linker detection and glibc version detection.
This also adds the improved logic using `/usr/bin/env` rather than
invoking the system C compiler to find the dynamic linker when zig
is statically linked. Related: #2084
Note: this `/usr/bin/env` code is work-in-progress.
* `-target-glibc` CLI option is removed in favor of the new `-target`
syntax. Example: `-target x86_64-linux-gnu.2.27`
closes#1907
