RISC-V: Add support for resumable non-maskable interrupt (RNMI) handlers

RISC-V: Add support for resumable non-maskable interrupt (RNMI) handlers

The Smrnmi extension introduces the nmret instruction to return from RNMI
handlers.  We already have basic Smrnmi support.  This patch introduces
support for the nmret instruction and the ability to set the function
attribute `__attribute__ ((interrupt ("rnmi")))` to let the compiler
generate RNMI handlers.

The attribute name is proposed in a PR for the RISC C API and approved
by LLVM maintainers:
  https://github.com/riscv-non-isa/riscv-c-api-doc/pull/116

gcc/ChangeLog:

	* config/riscv/riscv.cc (enum riscv_privilege_levels): Add
	RNMI_MODE.
	(riscv_handle_type_attribute): Handle 'rnmi' interrupt attribute.
	(riscv_expand_epilogue): Generate nmret for RNMI handlers.
	(riscv_get_interrupt_type): Handle 'rnmi' interrupt attribute.
	* config/riscv/riscv.md (riscv_rnmi): Add nmret INSN.
	* doc/extend.texi: Add documentation for 'rnmi' interrupt attribute.

gcc/testsuite/ChangeLog:

	* gcc.target/riscv/interrupt-rnmi.c: New test.

Signed-off-by: Christoph Müllner <christoph.muellner@vrull.eu>