Parse and emit hidden visibility

This adds parsing of (GCC compatible) visibility attribute
in order to mark selected global symbols as hidden.  The generated
.o files contain hidden symbols already, the TCC linker doesn't
yet do the right thing.

tcctok.h

@@ -121,6 +121,8 @@
      DEF(TOK_DLLIMPORT, "dllimport")
      DEF(TOK_NORETURN1, "noreturn")
      DEF(TOK_NORETURN2, "__noreturn__")
+     DEF(TOK_VISIBILITY1, "visibility")
+     DEF(TOK_VISIBILITY2, "__visibility__")
      DEF(TOK_builtin_types_compatible_p, "__builtin_types_compatible_p")
      DEF(TOK_builtin_constant_p, "__builtin_constant_p")
      DEF(TOK_builtin_frame_address, "__builtin_frame_address")