RISC-V: Add interrupt attribute modes.

RISC-V: Add interrupt attribute modes.

	gcc/
	* config/riscv/riscv.c (enum riscv_privilege_levels): New.
	(struct machine_function): New field interrupt_mode.
	(riscv_handle_type_attribute): New function.  Add forward declaration.
	(riscv_attribute_table) <interrupt>: Use riscv_handle_type_attribute.
	(riscv_expand_epilogue): Check interrupt_mode field.
	(riscv_set_current_function): Check interrupt attribute args and
	set interrupt_mode field.
	* config/riscv/riscv.md (UNSPECV_SRET, UNSPECV_URET): New.
	(riscv_sret, riscv_uret): New.
	* doc/extend.texi (RISC-V Function Attributes) <interrupt>: Document
	new arguments to interrupt attribute.

	gcc/testsuite/
	* gcc.target/riscv/interrupt-5.c (sub3): Add new test.
	* gcc.target/riscv/interrupt-mmode.c: New.
	* gcc.target/riscv/interrupt-smode.c: New.
	* gcc.target/riscv/interrupt-umode.c: New.

From-SVN: r261244