arm: Clear canary value after stack_protect_test [PR96191]

arm: Clear canary value after stack_protect_test [PR96191]

The stack_protect_test patterns were leaving the canary value in the
temporary register, meaning that it was often still in registers on
return from the function.  An attacker might therefore have been
able to use it to defeat stack-smash protection for a later function.

gcc/
	PR target/96191
	* config/arm/arm.md (arm_stack_protect_test_insn): Zero out
	operand 2 after use.
	* config/arm/thumb1.md (thumb1_stack_protect_test_insn): Likewise.

gcc/testsuite/
	* gcc.target/arm/stack-protector-1.c: New test.
	* gcc.target/arm/stack-protector-2.c: Likewise.