Reduce header complexity

- Remove most __ASSEMBLER__ __LINKER__ ifdefs
- Rename libc/intrin/bits.h to libc/serialize.h
- Block pthread cancelation in fchmodat() polyfill
- Remove `clang-format off` statements in third_party

libc/thread/pt.internal.h

@@ -0,0 +1,12 @@
+#ifndef COSMOPOLITAN_LIBC_THREAD_PT_H_
+#define COSMOPOLITAN_LIBC_THREAD_PT_H_
+
+#define PT_OWNSTACK       1
+#define PT_STATIC         2
+#define PT_ASYNC          4
+#define PT_NOCANCEL       8
+#define PT_MASKED         16
+#define PT_INCANCEL       32
+#define PT_OPENBSD_KLUDGE 64
+
+#endif /* COSMOPOLITAN_LIBC_THREAD_PT_H_ */