[arm] Fix regression by Do not skip prologue for asm (.S) files

[arm] Fix regression by Do not skip prologue for asm (.S) files

Patch "Do not skip prologue for asm (.S) files" [1] changes GDB's
behaviour on which test gdb.arch/thumb-singlestep.exp depends, so
it causes the fail below:

 (gdb) si^M
 37              blx     foo^M
 (gdb) FAIL: gdb.arch/thumb-singlestep.exp: step into foo

the test assumes the program will stop at the instruction after "push"
but it doesn't.  The fix to this fail is to do one more single step.

[1] https://sourceware.org/ml/gdb-patches/2015-06/msg00561.html

gdb/testsuite:

2015-07-07  Yao Qi  <yao.qi@linaro.org>

	* gdb.arch/thumb-singlestep.exp: Do one more single step.