testsuite: Fix gcc.target/arm/stack-protector-1.c for Cortex-M

The stack-protector-1.c test fails when compiled for Cortex-M:
- for Cortex-M0/M1, str r0, [sp #-8]! is not supported
- for Cortex-M3/M4..., the assembler complains that "use of r13 is
  deprecated"

This patch replaces the str instruction with
     sub   sp, sp, #8
     str r0, [sp]
and removes the check for r13, which is unlikely to leak the canary
value.

2020-08-11  Christophe Lyon  <christophe.lyon@linaro.org>

	gcc/testsuite/
	* gcc.target/arm/stack-protector-1.c: Adapt code to Cortex-M
	restrictions.