tcg: define tcg_init_ctx and make tcg_ctx a pointer

Groundwork for supporting multiple TCG contexts.

The core of this patch is this change to tcg/tcg.h:

> -extern TCGContext tcg_ctx;
> +extern TCGContext tcg_init_ctx;
> +extern TCGContext *tcg_ctx;

Note that for now we set *tcg_ctx to whatever TCGContext is passed
to tcg_context_init -- in this case &tcg_init_ctx.

Reviewed-by: Richard Henderson <rth@twiddle.net>
Signed-off-by: Emilio G. Cota <cota@braap.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
diff --git a/bsd-user/main.c b/bsd-user/main.c
index 836daac..392c0ed 100644
--- a/bsd-user/main.c
+++ b/bsd-user/main.c
@@ -977,7 +977,7 @@
     /* Now that we've loaded the binary, GUEST_BASE is fixed.  Delay
        generating the prologue until now so that the prologue can take
        the real value of GUEST_BASE into account.  */
-    tcg_prologue_init(&tcg_ctx);
+    tcg_prologue_init(tcg_ctx);
 
     /* build Task State */
     memset(ts, 0, sizeof(TaskState));