[ARM] Read memory as unsigned integer

[ARM] Read memory as unsigned integer

When GDB read inferior memory as an address or an instruction,
it should be unsigned.

gdb:

2016-11-30  Yao Qi  <yao.qi@linaro.org>

	* arm-tdep.c (arm_scan_prologue): Read memory as unsigned integer.
	(arm_exidx_unwind_sniffer): Likewise.