Use standard_testfile in gdb.arch/thumb-prologue.exp and gdb.arch/thumb2-it.exp

This patch fixes the errors below:

Running /home/yao/SourceCode/gnu/gdb/git/gdb/testsuite/gdb.arch/thumb-prologue.exp ...
gdb compile failed, arm-linux-gnueabihf/bin/ld: cannot open output file /scratch/yao/gdb/build-git/arm-linux-gnueabihf/gdb/testsuite/gdb.arch/thumb-prologue: No such file or directory
collect2: error: ld returned 1 exit status
Running /home/yao/SourceCode/gnu/gdb/git/gdb/testsuite/gdb.arch/thumb2-it.exp ...
gdb compile failed, arm-linux-gnueabihf/bin/ld: cannot open output file /scratch/yao/gdb/build-git/arm-linux-gnueabihf/gdb/testsuite/gdb.arch/thumb2-it: No such file or directory

gdb/testsuite:

2016-05-23  Yao Qi  <yao.qi@linaro.org>

	* gdb.arch/thumb-prologue.exp: Use standard_testfile.
	* gdb.arch/thumb2-it.exp: Likewise.

gdb/testsuite/ChangeLog

@@ -1,3 +1,8 @@
+2016-05-23  Yao Qi  <yao.qi@linaro.org>
+
+	* gdb.arch/thumb-prologue.exp: Use standard_testfile.
+	* gdb.arch/thumb2-it.exp: Likewise.
+
 2016-05-18  Simon Marchi  <simon.marchi@ericsson.com>
 
 	* gdb.mi/mi-threads-interrupt.c: New file.

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

@@ -20,9 +20,7 @@ if {![is_aarch32_target]} then {
     return
 }
 
-set testfile "thumb-prologue"
-set srcfile ${testfile}.c
-set binfile ${objdir}/${subdir}/${testfile}
+standard_testfile
 
 # Don't use "debug", so that we don't have line information for the assembly
 # fragments.

gdb/testsuite/gdb.arch/thumb2-it.exp

@@ -20,9 +20,7 @@ if {![istarget arm*-*eabi*]} then {
     return
 }
 
-set testfile "thumb2-it"
-set srcfile ${testfile}.S
-set binfile ${objdir}/${subdir}/${testfile}
+standard_testfile .S
 
 if { [gdb_compile "${srcdir}/${subdir}/${srcfile}" "${binfile}" executable debug] != "" } {
     untested thumb2-it.exp