Use ELF_STRING_ARM_unwind in arm-tdep.c

Use ELF_STRING_ARM_unwind in arm-tdep.c

We've already has the definition like this,

 #define ELF_STRING_ARM_unwind           ".ARM.exidx"

so it is better to use the macro rather than the string.

gdb:

2015-11-09  Yao Qi  <yao.qi@linaro.org>

	* arm-tdep.c (arm_exidx_new_objfile): Use
	ELF_STRING_ARM_unwind.