S390: Fix output path for s390-tdbregs test case

Since test artifacts are always organized in a directory hierarchy, the
s390-tdbregs test case is not executed correctly any more.  This is
because it uses an obsolete way of constructing the executable's path.

This change invokes prepare_for_testing instead.

gdb/testsuite/ChangeLog:

	* gdb.arch/s390-tdbregs.exp: Use prepare_for_testing instead of
	manually constructing the output path.

gdb/testsuite/ChangeLog

@@ -1,3 +1,8 @@
+2016-03-01  Andreas Arnez  <arnez@linux.vnet.ibm.com>
+
+	* gdb.arch/s390-tdbregs.exp: Use prepare_for_testing instead of
+	manually constructing the output path.
+
 2016-03-01  Andreas Arnez  <arnez@linux.vnet.ibm.com>
 
 	* gdb.arch/s390-stackless.S: New.

gdb/testsuite/gdb.arch/s390-tdbregs.exp

@@ -24,25 +24,12 @@ if { ![istarget s390-*-*] && ![istarget s390x-*-* ] } {
     return
 }
 
-set testfile "s390-tdbregs"
-set srcfile ${testfile}.c
-set binfile ${objdir}/${subdir}/${testfile}
+standard_testfile .c
 
-if { [get_compiler_info] } {
+if { [prepare_for_testing $testfile.exp $testfile $srcfile] } {
     return -1
 }
 
-if { [gdb_compile "${srcdir}/${subdir}/${srcfile}" "${binfile}" \
-      executable [list debug]] != "" } {
-    fail "compile failed"
-    return
-}
-
-gdb_exit
-gdb_start
-gdb_reinitialize_dir $srcdir/$subdir
-gdb_load ${binfile}
-
 if { ![runto_main] } then {
     gdb_suppress_tests
 }