[ion][python] Implement an architecture-dependant collect_registers.

setjmp is not guaranteed to collect all registers without modification
on all platforms.

This fixes the following bug: when the pointer of a newly allocated
object on the Python heap is stored in rpb registers on x86_64 arch, it
was not collected by the garbarge collector.

ion/include/ion.h

@@ -17,6 +17,7 @@
 #include <ion/unicode/utf8_helper.h>
 #include <stdint.h>
 #include <string.h>
+#include <setjmp.h>
 
 /* ION is not your regular library. It is a library you link against, but it
  * will take care of configuring the whole environment for you. In POSIX terms,
@@ -48,6 +49,9 @@ void decompress(const uint8_t * src, uint8_t * dst, int srcSize, int dstSize);
 // Tells whether the stack pointer is within acceptable bounds
 bool stackSafe();
 
+// Collect registers in a buffer and returns the stack pointer
+uintptr_t collectRegisters(jmp_buf regs);
+
 }
 
 #endif