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, gcc-mirror#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.

(cherry picked from commit 6606fdc)

Author: Christophe Lyon

gcc/testsuite/gcc.target/arm/stack-protector-1.c

@@ -34,7 +34,8 @@ asm (
 "	.type	main, %function\n"
 "main:\n"
 "	bl	get_ptr\n"
-"	str	r0, [sp, #-8]!\n"
+"	sub	sp, sp, #8\n"
+"	str	r0, [sp]\n"
 "	bl	f\n"
 "	str	r0, [sp, #4]\n"
 "	ldr     r0, [sp]\n"
@@ -51,7 +52,6 @@ asm (
 	CHECK (r10)
 	CHECK (r11)
 	CHECK (r12)
-	CHECK (r13)
 	CHECK (r14)
 "	ldr	r1, [sp, #4]\n"
 	CHECK (r1)