[ARM] Fix copysign_softfloat_1.c option directives

[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