introduce operating system version ranges as part of the target

* 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

lib/std/packed_int_array.zig

@@ -593,7 +593,7 @@ test "PackedInt(Array/Slice)Endian" {
 // after this one is not mapped and will cause a segfault if we
 // don't account for the bounds.
 test "PackedIntArray at end of available memory" {
-    switch (builtin.os) {
+    switch (builtin.os.tag) {
         .linux, .macosx, .ios, .freebsd, .netbsd, .windows => {},
         else => return,
     }
@@ -612,7 +612,7 @@ test "PackedIntArray at end of available memory" {
 }
 
 test "PackedIntSlice at end of available memory" {
-    switch (builtin.os) {
+    switch (builtin.os.tag) {
         .linux, .macosx, .ios, .freebsd, .netbsd, .windows => {},
         else => return,
     }