RISC-V: Add interrupt attribute support.

RISC-V: Add interrupt attribute support.

	gcc/
	* config/riscv/riscv-protos.h (riscv_epilogue_uses): New.
	* config/riscv/riscv.c (struct machine_function): Add
	interrupt_handler_p and attribute_checked_p fields.
	(riscv_attribute_table): Add interrupt.
	(riscv_interrupt_type_p): New.
	(riscv_save_reg_p): Save extra regs for interrupt handler.
	(riscv_use_save_libcall): Return false  for interrupt handler.
	(riscv_first_stack_step): Add forward declaration.
	(riscv_compute_frame_info): New local interrupt_save_t1.  Set it
	for interrupt handler with large frame.  Use it for saved reg list.
	(riscv_expand_prologue): Move flag_stack_usage_info support to
	eliminate duplication.
	(riscv_expand_epilogue): Generate mret for interrupt handler.
	(riscv_epilogue_uses): New.
	(riscv_can_use_return_insn): Return false for interrupt handler.
	(riscv_function_ok_for_sibcall): Likewise.
	(riscv_set_current_function): Add interrupt handler support.
	* config/riscv/riscv.h (EPILOGUE_USES): Call riscv_epilogue_uses.
	* config/riscv/riscv.md (UNSPECV_MRET): New.
	(GP_REGNUM): New.
	(riscv_frflags, riscv_fsflags): Use tab after opcode.
	(riscv_mret): New.
	* doc/extend.texi (RISC-V Function Attributes) <interrupt>: New.

	gcc/testsuite/
	* gcc.target/riscv/interrupt-1.c: New.
	* gcc.target/riscv/interrupt-2.c: New.
	* gcc.target/riscv/interrupt-3.c: New.
	* gcc.target/riscv/interrupt-4.c: New.
	* gcc.target/riscv/interrupt-5.c: New.

From-SVN: r260785