[ARM] Fix copysign_softfloat_1.c option directives

gcc.target/arm/copysign_softfloat_1.c's use of arm_arch_v6t2 in
dg-add-option changes the architecture to -march=armv6t2. Since the test
only requires Thumb-2 capable architecture, we just need to add -mthumb
on the command line since arm_thumb2_ok guarantees by definition that
doing that is enough to select Thumb-2. This fixes warning on the
command line when having -mcpu=cortex-m3 in RUNTESTFLAGS for instance.

2018-03-01  Thomas Preud'homme  <thomas.preudhomme@arm.com

    gcc/testsuite/
    * gcc.target/arm/copysign_softfloat_1.c: Remove dg-add-options and add
    -mthumb to the command-line.

From-SVN: r258108

gcc/testsuite/ChangeLog

+2018-03-01  Thomas Preud'homme  <thomas.preudhomme@arm.com
+
+	* gcc.target/arm/copysign_softfloat_1.c: Remove dg-add-options and add
+	-mthumb to the command-line.
+
 2018-03-01  Marek Polacek  <polacek@redhat.com>
 
 	PR c++/84596

gcc/testsuite/gcc.target/arm/copysign_softfloat_1.c

 /* { dg-do run } */
 /* { dg-require-effective-target arm_thumb2_ok } */
-/* { dg-add-options arm_arch_v6t2 } */
-/* { dg-additional-options "-O2 --save-temps" } */
+/* { dg-additional-options "-mthumb -O2 --save-temps" } */
 
 extern void abort (void);