tests: OOT build fixes etc.

tests/Makefile: fix out-of-tree build issues

Also:

- win64: align(16) MEM_DEBUG user memory
  on win64 the struct jmp_buf in the TCCState structure which we
  allocate by tcc_malloc needs alignment 16 because the msvcrt
  setjmp uses MMX instructions.

- libtcc_test.c: win32/64 need __attribute__((dllimport)) for
  extern data objects

- tcctest.c: exclude stuff that gcc does not compile
  except for relocation_test() the other issues are mostly ASM
  related.  We should probably check GCC versions but I have
  no idea which mingw/gcc versions support what and which don't.

- lib/Makefile: use tcc to compile libtcc1.a (except on arm
  which needs arm-asm

libtcc.c

@@ -262,7 +262,7 @@ struct mem_debug_header {
     int line_num;
     char file_name[MEM_DEBUG_FILE_LEN + 1];
     unsigned magic2;
-    unsigned magic3;
+    __attribute__((aligned(16))) unsigned magic3;
 };
 
 typedef struct mem_debug_header mem_debug_header_t;