target/arm: Fix outdated comment about exception exit

When we switched our handling of exception exit to detect
the magic addresses at translate time rather than via
a do_unassigned_access hook, we forgot to update a
comment; correct the omission.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Edgar E. Iglesias <edgar.iglesias@xilinx.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 1501692241-23310-8-git-send-email-peter.maydell@linaro.org

target/arm/helper.c

@@ -6138,7 +6138,7 @@ static void do_v7m_exception_exit(ARMCPU *cpu)
     bool rettobase = false;
 
     /* We can only get here from an EXCP_EXCEPTION_EXIT, and
-     * arm_v7m_do_unassigned_access() enforces the architectural rule
+     * gen_bx_excret() enforces the architectural rule
      * that jumps to magic addresses don't have magic behaviour unless
      * we're in Handler mode (compare pseudocode BXWritePC()).
      */