tcg: Add tcg_out_tb_start backend hook

This hook may emit code at the beginning of the TB.

Suggested-by: Jordan Niethe <jniethe5@gmail.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>

tcg/arm/tcg-target.c.inc

@@ -2962,6 +2962,11 @@ static void tcg_out_epilogue(TCGContext *s)
                   (1 << TCG_REG_R10) | (1 << TCG_REG_R11) | (1 << TCG_REG_PC));
 }
 
+static void tcg_out_tb_start(TCGContext *s)
+{
+    /* nothing to do */
+}
+
 typedef struct {
     DebugFrameHeader h;
     uint8_t fde_def_cfa[4];