RISC-V: Fix incorrect test macro for signed scalar SAT_ADD form 2 run test

This patch would like to fix one incorrect test macro usage for
form 2 of signed scalar SAT_ADD run test.  It should leverage the
_FMT_2 instead of _FMT_1 for form 2.

The below test are passed for this patch.
* The rv64gcv fully regression test.

It is test only patch and obvious up to a point, will commit it
directly if no comments in next 48H.

gcc/testsuite/ChangeLog:

	* gcc.target/riscv/sat_arith.h: Add test helper macro.
	* gcc.target/riscv/sat_s_add-run-5.c: Take form 2 for run test.
	* gcc.target/riscv/sat_s_add-run-6.c: Ditto.
	* gcc.target/riscv/sat_s_add-run-7.c: Ditto.
	* gcc.target/riscv/sat_s_add-run-8.c: Ditto.

Signed-off-by: Pan Li <pan2.li@intel.com>