OpenBSD: runtime fixes

After this commit we can compile and run code with some limitations.
- The dlsym function is broken so this makes -run and bound checking
  not work all the time. Make -k test does work for most code.
- You have to do:
    ln -s /usr/lib/libN.so.x.y /usr/lib/libN.so
  for all .so files in /usr/lib.
  OpenBSD uses opendir/readdir to find the correct so file. This is
  not the way other platforms do this.
  Also the .a versions do not have all symbols that are present in the .so
  files.

tcc.h:
- Use different dynamic loader

elf.h:
- Add SHT_X86_64_UNWIND

tccelf.c:
- Do not use -dl
- Add required NOTE section
- Add extra dynamic tags
- Allow SHT_X86_64_UNWIND/SHT_NOTE in tcc_load_object_file

tccrun.c:
- Uses MAP_FIXED because without the offset between exec and data section
  becomes too big for x86_64

lib/bcheck.c:
- Do not use __libc_freeres

tests/tcctest.c:
- aligned_function also disabled for __GNUC__ == 4

lib/bcheck.c

@@ -1147,7 +1147,7 @@ void __attribute__((destructor)) __bound_exit(void)
     dprintf(stderr, "%s, %s():\n", __FILE__, __FUNCTION__);
 
     if (inited) {
-#if !defined(_WIN32) && !defined(__APPLE__)
+#if !defined(_WIN32) && !defined(__APPLE__) && !defined(__OpenBSD__)
         if (print_heap) {
             extern void __libc_freeres (void);
             __libc_freeres ();