Apply stub unwinder to 'bx reg' trampoline

Apply stub unwinder to 'bx reg' trampoline

In target arm-none-eabi, prologue unwinder is used for trampoline
'bx reg'.  However, in target arm-linux, exidx unwinder is selected for
trampoline at first, which is not expected.  The main function and the
trampoline is,

   0x00009dfc <main+0>: push    {r4, r5, r6, r7, lr}
   ......
   0x0000ac30 <main+3636>:      ldrdeq  r3, [r1], -r8
   0x0000ac34:  bx      r2
   0x0000ac36:  bx      r4

and .ARM.exidx is:

0x9dfc <main>: @0xb404
  Compact model index: 1
  0x97      vsp = r7
  0x20      vsp = vsp + 132
  0x3f      vsp = vsp + 256
  0x80 0xf0 pop {r8, r9, r10, r11}
  0xab      pop {r4, r5, r6, r7, r14}

0xac38 <__aeabi_drsub>: 0x1 [cantunwind]

Trampolines 'bx r2' and 'bx r4' doesn't belong to main, but the exidx
for main is still selected form them because there is no end address
of each exidx entry.

Instead of teaching exidx unwinder ignore this trampoline (which looks
complicated and error prone), I decide to let stub unwinder to handle
trampoline, because stub undwinder is installed before exidx unwinder,
and this trampoline can be regarded as a stub too.

This patch is to add the code to match 'bx reg' trampoline in the
sniffer of stub unwinder.

gdb:

2014-06-24  Yao Qi  <yao@codesourcery.com>

	* arm-tdep.c (arm_stub_unwind_sniffer): Return 1 if
	arm_skip_bx_reg returns	non-zero.