[OPCODE][AARCH64]Check mapping symbol while backward searching for IT block

[Nick Clifton]

Hi Renlin,

> Here is the patch to make the test to skip non-elf target.
> The test case here is only applicable to elf target which has mapping symbol defined.

> gas/ChangeLog:
> 2016-01-28  Renlin Li  <renlin.li@arm.com>
>
>      * testsuite/gas/arm/thumb2_it_search.d: Skip non-elf targets.

Approved - please apply.

Cheers
   Nick