MINOR: compiler: add a new __decl_thread_var() macro to declare local variables

MINOR: compiler: add a new __decl_thread_var() macro to declare local variables

__decl_thread() already exists but is more suited for struct members.
When using it in a variables block, it appends the final trailing
semi-colon which is a statement that ends the variable block. Better
clean this up and have one precisely for variable blocks. In this
case we can simply define an unused enum value that will consume the
semi-colon. That's what the new macro __decl_thread_var() does.

(cherry picked from commit 69ac4cd315f05c69f3c34f824ef4e7f3221966cf)
Signed-off-by: Willy Tarreau <w@1wt.eu>