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

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:

2015-12-02  Yao Qi  <yao.qi@linaro.org>

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