RISC-V: Fix testcase failed when default -mcmodel=medany

RISC-V: Fix testcase failed when default -mcmodel=medany

This patch fix testcase failed when I build RISC-V GCC with -mcmodel=medany
as default. If set to medany, stack_save_restore.c testcase will fail because of
the reduced use of s3 registers in assembly (thus calling __riscv_save/store_3
instead of __riscv_save/store_4). So relax assert from
`__riscv_save/restore_4` to `__riscv_save/restore_(3|4)` to let this
testcase not brittle on any -mcmodel and add another testcase that use
-march=rv64imafc.

gcc/testsuite/ChangeLog:

	* gcc.target/riscv/stack_save_restore.c: Moved to...
	* gcc.target/riscv/stack_save_restore_2.c: ...here.
	* gcc.target/riscv/stack_save_restore_1.c: New test.