[arm] gcc.target/arm/cs*: Use dg-add-options arm_arch_v8_1m_main

[arm] gcc.target/arm/cs*: Use dg-add-options arm_arch_v8_1m_main

These testcases need thumb mode, which may not be the default.

Using dg-add-options arm_arch_v8_1m_main ensures that -mthumb is used
and makes the test pass in more configurations.

2020-09-21  Christophe Lyon  <christophe.lyon@linaro.org>

	gcc/testsuite/
	* gcc.target/arm/csinc-1.c: Use dg-add-options
	arm_arch_v8_1m_main.
	* gcc.target/arm/csinv-1.c: Likewise.
	* gcc.target/arm/csneg.c: Likewise.