match glibc/lsb cancellation abi on i386

glibc made the ridiculous choice to use pass-by-register calling
convention for these functions, which is impossible to duplicate
directly on non-gcc compilers. instead, we use ugly asm to wrap and
convert the calling convention. presumably this works with every
compiler anyone could potentially want to use.

src/thread/pthread_create.c

@@ -1,5 +1,10 @@
 #include "pthread_impl.h"
 
+#ifdef __pthread_unwind_next
+#undef __pthread_unwind_next
+#define __pthread_unwind_next __pthread_unwind_next_3
+#endif
+
 void __pthread_unwind_next(struct __ptcb *cb)
 {
 	int i, j, not_finished;