std.Target: Remove functions that just wrap component functions.

Functions like isMinGW() and isGnuLibC() have a good reason to exist: They look
at multiple components of the target. But functions like isWasm(), isDarwin(),
isGnu(), etc only exist to save 4-8 characters. I don't think this is a good
enough reason to keep them, especially given that:

* It's not immediately obvious to a reader whether target.isDarwin() means the
  same thing as target.os.tag.isDarwin() precisely because isMinGW() and similar
  functions *do* look at multiple components.
* It's not clear where we would draw the line. The logical conclusion before
  this commit would be to also wrap Arch.isX86(), Os.Tag.isSolarish(),
  Abi.isOpenHarmony(), etc... this obviously quickly gets out of hand.
* It's nice to just have a single correct way of doing something.

lib/std/Thread.zig

@@ -734,7 +734,7 @@ const PosixThreadImpl = struct {
             else => {
                 var count: c_int = undefined;
                 var count_len: usize = @sizeOf(c_int);
-                const name = if (comptime target.isDarwin()) "hw.logicalcpu" else "hw.ncpu";
+                const name = if (comptime target.os.tag.isDarwin()) "hw.logicalcpu" else "hw.ncpu";
                 posix.sysctlbynameZ(name, &count, &count_len, null, 0) catch |err| switch (err) {
                     error.NameTooLong, error.UnknownName => unreachable,
                     else => |e| return e,