arm: Fix ICE with -fstack-protector -mpure-code [PR98998]

arm: Fix ICE with -fstack-protector -mpure-code [PR98998]

The vla15.C testcase ICEs with
-mcpu=cortex-m1 -mpure-code -fstack-protector -mthumb
as what force_const_mem returns (a SYMBOL_REF) is not a valid
memory address.
Previously the code was moving the address of the force_const_mem
into a register rather than the content of that MEM, so that instruction
must have been supported and loading from a MEM with a single REG base ought
to be valid too.

2021-02-19  Jakub Jelinek  <jakub@redhat.com>

	PR target/98998
	* config/arm/arm.md (*stack_protect_combined_set_insn,
	*stack_protect_combined_test_insn): If force_const_mem result
	is not valid general operand, force its address into the destination
	register first.

	* gcc.target/arm/pure-code/pr98998.c: New test.