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

test/libc/calls/sigprocmask_test.c

@@ -23,11 +23,12 @@
 #include "libc/calls/struct/sigset.h"
 #include "libc/calls/ucontext.h"
 #include "libc/dce.h"
+#include "libc/intrin/kprintf.h"
 #include "libc/sysv/consts/sa.h"
 #include "libc/sysv/consts/sig.h"
 #include "libc/testlib/testlib.h"
 
-int n;
+volatile int n;
 
 void OnSig(int sig, siginfo_t *si, ucontext_t *ctx) {
   ++n;