[arm] Fix fails in gdb.base/disp-step-syscall.exp

[arm] Fix fails in gdb.base/disp-step-syscall.exp

Hi,
I see this fail on arm-linux target,

 FAIL: gdb.base/disp-step-syscall.exp: fork: single step over fork final pc

which is caused by the PC isn't expected after displaced stepping the
svc instruction.  The code is:

=> 0xb6ead9a4 <__libc_do_syscall+4>:    svc     0
   0xb6ead9a6 <__libc_do_syscall+6>:    pop     {r7, pc}
   0xb6ead9a8:  nop.w^M
   0xb6ead9ac:  nop.w

after single step svc instruction, pc should be 0xb6ead9a6, but the
actual value of pc is 0xb6ead9a8.  The problem is illustrated by
turning on debug message of displaced stepping,

stepi^M
displaced: stepping Thread 12031 now^M
displaced: saved 0x8574: 02 bc 6a 46 04 b4 01 b4 df f8 10 c0 4d f8 04 cd 03 48 04 4b ff f7 d2 ef ff f7 e8 ef 0d 87 00 00 ^M
displaced: process thumb insn df00 at b6ead9a4^M
displaced: copying svc insn df00^M
displaced: read r7 value 00000078^M
displaced: sigreturn/rt_sigreturn SVC call not in signal trampoline frame^M
displaced: writing insn df00 at 00008574^M
displaced: copy 0xb6ead9a4->0x8574: displaced: check mode of b6ead9a4 instead of 00008574^M
displaced: displaced pc to 0x8574^M
displaced: run 0x8574: 00 df 01 de ^M
displaced: restored Thread 12031 0x8574^M
displaced: PC is apparently 00008576 after SVC step (within scratch space)^M
displaced: writing pc b6ead9a8  <----- WRONG ADDRESS

GDB writes the wrong address back to pc because GDB thinks the
instruction size is 4, which isn't true for thumb instruction.
This patch is to replace 4 with dsc->insn_size.

gdb:

2015-04-15  Yao Qi  <yao.qi@linaro.org>

	* arm-linux-tdep.c (arm_linux_cleanup_svc): Use
	dsc->insn_size instead of 4.