Improve locks and signals

- Introduce fast spinlock API
- Double rand64() perf w/ spinlock
- Improve raise() on New Technology
- Support gettid() across platforms
- Implement SA_NODEFER on New Technology
- Move the lock intrinsics into LIBC_INTRIN
- Make SIGTRAP recoverable on New Technology
- Block SIGCHLD in wait4() on New Technology
- Add threading prototypes for XNU and FreeBSD
- Rewrite abort() fixing its minor bugs on XNU/NT
- Shave down a lot of the content in libc/bits/bits.h
- Let signal handlers modify CPU registers on New Technology

libc/calls/reservefd.c

@@ -16,8 +16,8 @@
 │ TORTIOUS ACTION, ARISING OUT OF OR IN CONNECTION WITH THE USE OR             │
 │ PERFORMANCE OF THIS SOFTWARE.                                                │
 ╚─────────────────────────────────────────────────────────────────────────────*/
-#include "libc/bits/bits.h"
 #include "libc/calls/internal.h"
+#include "libc/intrin/cmpxchg.h"
 #include "libc/mem/mem.h"
 #include "libc/sysv/errfuns.h"
 
@@ -31,8 +31,8 @@ int __reservefd(void) {
     if (fd >= g_fds.n) {
       if (__ensurefds(fd) == -1) return -1;
     }
-    cmpxchg(&g_fds.f, fd, fd + 1);
-    if (cmpxchg(&g_fds.p[fd].kind, kFdEmpty, kFdReserved)) {
+    _cmpxchg(&g_fds.f, fd, fd + 1);
+    if (_cmpxchg(&g_fds.p[fd].kind, kFdEmpty, kFdReserved)) {
       return fd;
     }
   }