RISC-V: Add Zicfilp ISA extension.

RISC-V: Add Zicfilp ISA extension.

This patch only support landing pad value is 0.
The next version will implement function signature based labeling
scheme.

RISC-V CFI SPEC: https://github.com/riscv/riscv-cfi

gcc/ChangeLog:
	* common/config/riscv/riscv-common.cc: Add ZICFILP ISA
	string.
	* config.gcc: Add riscv-zicfilp.o
	* config/riscv/riscv-passes.def (INSERT_PASS_BEFORE):
	Insert landing pad instructions.
	* config/riscv/riscv-protos.h (make_pass_insert_landing_pad):
	Declare.
	* config/riscv/riscv-zicfilp.cc: New file.
	* config/riscv/riscv.cc
	(riscv_trampoline_init): Add landing pad instructions.
	(riscv_legitimize_call_address): Likewise.
	(riscv_output_mi_thunk): Likewise.
	* config/riscv/riscv.h: Update.
	* config/riscv/riscv.md: Add landing pad patterns.
	* config/riscv/riscv.opt (TARGET_ZICFILP): Define.
	* config/riscv/t-riscv: Add build rule for
	riscv-zicfilp.o

gcc/testsuite/ChangeLog:
	* gcc.target/riscv/interrupt-no-lpad.c: New test.
	* gcc.target/riscv/zicfilp-call.c: New test.

	Co-Developed-by: Greg McGary <gkm@rivosinc.com>,
			Kito Cheng <kito.cheng@gmail.com>