[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.

gdb/testsuite/ChangeLog

@@ -1,3 +1,7 @@
+2015-07-07  Yao Qi  <yao.qi@linaro.org>
+
+	* gdb.arch/thumb-singlestep.exp: Do one more single step.
+
 2015-06-30  Martin Galvan  <martin.galvan@tallertechnologies.com>
 
 	* lib/gdb.exp (test_class_help): Remove the unneeded escaping of

gdb/testsuite/gdb.arch/thumb-singlestep.exp

@@ -34,5 +34,8 @@ if ![runto_main] then {
     return -1
 }
 
+# GDB doesn't skip prologue for asm files, so do one single step to
+# pass instruction "push".
+gdb_test "si" "blx	foo.*" "single step"
 gdb_test "si" "foo \\(\\) at .*${srcfile}.*mov r0,#42.*" "step into foo"