Revert "Add support for thread-local storage variables"

TLS support in tinyCC is absolutely not ready:
- segment register not select in load and store
- no relocation added for computing offset of per-thread symbol
- no support for TLS-specific relocations
- no program header added as per Drepper document about TLS

This reverts commit 1c4afd1350.

tcctok.h

@@ -10,7 +10,6 @@
      DEF(TOK_FOR, "for")
      DEF(TOK_EXTERN, "extern")
      DEF(TOK_STATIC, "static")
-     DEF(TOK_THREAD, "__thread")
      DEF(TOK_UNSIGNED, "unsigned")
      DEF(TOK_GOTO, "goto")
      DEF(TOK_DO, "do")