x86-64: shared libs improvement

This correctly resolves local references to global functions from
shared libs to their PLT slot (instead of directly to the target
symbol), so that interposition works.

This is still not 100% conforming (executables don't export symbols
that are also defined in linked shared libs, as they must), but
normal shared lib situations work.

tcc.h

@@ -522,7 +522,7 @@ typedef struct ASMOperand {
 
 struct sym_attr {
     unsigned long got_offset;
-    unsigned char has_plt_entry:1;
+    unsigned long plt_offset;
 #ifdef TCC_TARGET_ARM
     unsigned char plt_thumb_stub:1;
 #endif