Make futexes 100x better on x86 MacOS

Thanks to @autumnjolitz (in #876) the Cosmopolitan codebase is now
acquainted with Apple's outstanding ulock system calls which offer
something much closer to futexes than Grand Central Dispatch which
wasn't quite as good, since its wait function can't be interrupted
by signals (therefore necessitating a busy loop) and it also needs
semaphore objects to be created and freed. Even though ulock is an
internal Apple API, strictly speaking, the benefits of futexes are
so great that it's worth the risk for now especially since we have
the GCD implementation still as a quick escape hatch if it changes

Here's why this change is important for x86 XNU users. Cosmo has a
suboptimal polyfill when the operating system doesn't offer an API
that let's us implement futexes properly. Sadly we had to use that
on X86 XNU until now. The polyfill works using clock_nanosleep, to
poll the futex in a busy loop with exponential backoff. On XNU x86
clock_nanosleep suffers from us not being able to use a fast clock
gettime implementation, which had a compounding effect that's made
the polyfill function even more poorly. On X86 XNU we also need to
polyfill sched_yield() using select(), which made things even more
troublesome. Now that we have futexes we don't have any busy loops
anymore for both condition variables and thread joining so optimal
performance is attained. To demonstrate, consider these benchmarks

Before:

    $ ./lockscale_test.com -b
    consumed 38.8377   seconds real time and
              0.087131 seconds cpu time

After:

    $ ./lockscale_test.com -b
    consumed 0.007955 seconds real time and
             0.011515 seconds cpu time

Fixes #876

libc/intrin/ulock.c

@@ -0,0 +1,54 @@
+/*-*- mode:c;indent-tabs-mode:nil;c-basic-offset:2;tab-width:8;coding:utf-8 -*-│
+│vi: set net ft=c ts=2 sts=2 sw=2 fenc=utf-8                                :vi│
+╞══════════════════════════════════════════════════════════════════════════════╡
+│ Copyright 2023 Justine Alexandra Roberts Tunney                              │
+│                                                                              │
+│ Permission to use, copy, modify, and/or distribute this software for         │
+│ any purpose with or without fee is hereby granted, provided that the         │
+│ above copyright notice and this permission notice appear in all copies.      │
+│                                                                              │
+│ THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL                │
+│ WARRANTIES WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED                │
+│ WARRANTIES OF MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE             │
+│ AUTHOR BE LIABLE FOR ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL         │
+│ DAMAGES OR ANY DAMAGES WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR        │
+│ PROFITS, WHETHER IN AN ACTION OF CONTRACT, NEGLIGENCE OR OTHER               │
+│ TORTIOUS ACTION, ARISING OUT OF OR IN CONNECTION WITH THE USE OR             │
+│ PERFORMANCE OF THIS SOFTWARE.                                                │
+╚─────────────────────────────────────────────────────────────────────────────*/
+#include "libc/intrin/ulock.h"
+#include "libc/assert.h"
+#include "libc/calls/calls.h"
+#include "libc/calls/syscall_support-sysv.internal.h"
+#include "libc/dce.h"
+#include "libc/intrin/describeflags.internal.h"
+#include "libc/intrin/strace.internal.h"
+
+// XNU futexes
+// https://opensource.apple.com/source/xnu/xnu-7195.50.7.100.1/bsd/sys/ulock.h.auto.html
+// https://opensource.apple.com/source/xnu/xnu-3789.41.3/bsd/kern/sys_ulock.c.auto.html
+
+int sys_ulock_wait(uint32_t operation, void *addr, uint64_t value,
+                   uint32_t timeout_micros) asm("sys_futex_cp");
+
+// returns -1 w/ errno
+int ulock_wait(uint32_t operation, void *addr, uint64_t value,
+               uint32_t timeout_micros) {
+  int rc;
+  operation |= ULF_WAIT_CANCEL_POINT;
+  STRACE("ulock_wait(%#x, %p, %lx, %u) → ...", operation, addr, value,
+         timeout_micros);
+  rc = sys_ulock_wait(operation, addr, value, timeout_micros);
+  STRACE("ulock_wait(%#x, %p, %lx, %u) → %d% m", operation, addr, value,
+         timeout_micros, rc);
+  return rc;
+}
+
+// returns -errno
+int ulock_wake(uint32_t operation, void *addr, uint64_t wake_value) {
+  int rc;
+  rc = __syscall3i(operation, (long)addr, wake_value, 0x2000000 | 516);
+  STRACE("ulock_wake(%#x, %p, %lx) → %s", operation, addr, wake_value,
+         DescribeErrno(rc));
+  return rc;
+}