organize std lib concurrency primitives and add RwLock

* move concurrency primitives that always operate on kernel threads to
   the std.Thread namespace
 * remove std.SpinLock. Nobody should use this in a non-freestanding
   environment; the other primitives are always preferable. In
   freestanding, it will be necessary to put custom spin logic in there,
   so there are no use cases for a std lib version.
 * move some std lib files to the top level fields convention
 * add std.Thread.spinLoopHint
 * add std.Thread.Condition
 * add std.Thread.Semaphore
 * new implementation of std.Thread.Mutex for Windows and non-pthreads Linux
 * add std.Thread.RwLock

Implementations provided by @kprotty

src/Compilation.zig

@@ -125,7 +125,7 @@ owned_link_dir: ?std.fs.Dir,
 color: @import("main.zig").Color = .auto,
 
 /// This mutex guards all `Compilation` mutable state.
-mutex: std.Mutex = .{},
+mutex: std.Thread.Mutex = .{},
 
 test_filter: ?[]const u8,
 test_name_prefix: ?[]const u8,