LoongArch: testsuite: Disable stack protector for some tests

Stack protector will affect stack layout and break the expectation of
these tests, causing test failures if GCC is configured with
--enable-default-ssp.

gcc/testsuite/ChangeLog:

	* gcc.target/loongarch/prolog-opt.c (dg-options): Add
	-fno-stack-protector.
	* gcc.target/loongarch/stack-check-cfa-1.c (dg-options):
	Likewise.
	* gcc.target/loongarch/stack-check-cfa-2.c (dg-options):
	Likewise.