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

lib/std/debug.zig

@@ -50,7 +50,7 @@ pub const LineInfo = struct {
     }
 };
 
-var stderr_mutex = std.Mutex{};
+var stderr_mutex = std.Thread.Mutex{};
 
 /// Deprecated. Use `std.log` functions for logging or `std.debug.print` for
 /// "printf debugging".
@@ -65,7 +65,7 @@ pub fn print(comptime fmt: []const u8, args: anytype) void {
     nosuspend stderr.print(fmt, args) catch return;
 }
 
-pub fn getStderrMutex() *std.Mutex {
+pub fn getStderrMutex() *std.Thread.Mutex {
     return &stderr_mutex;
 }
 
@@ -235,7 +235,7 @@ pub fn panic(comptime format: []const u8, args: anytype) noreturn {
 var panicking: u8 = 0;
 
 // Locked to avoid interleaving panic messages from multiple threads.
-var panic_mutex = std.Mutex{};
+var panic_mutex = std.Thread.Mutex{};
 
 /// Counts how many times the panic handler is invoked by this thread.
 /// This is used to catch and handle panics triggered by the panic handler.
@@ -280,7 +280,7 @@ pub fn panicExtra(trace: ?*const builtin.StackTrace, first_trace_addr: ?usize, c
                 // and call abort()
 
                 // Sleep forever without hammering the CPU
-                var event: std.StaticResetEvent = .{};
+                var event: std.Thread.StaticResetEvent = .{};
                 event.wait();
                 unreachable;
             }