RISC-V: Add Zfbfmin extension

1 In the previous patch, the libcall for BF16 was implemented:
<https://gcc.gnu.org/git/?p=gcc.git;a=commit;h=8c7cee80eb50792e57d514be1418c453ddd1073e>

2 Riscv provides Zfbfmin extension, which completes the "Scalar BF16 Converts":
<https://github.com/riscv/riscv-bfloat16/blob/main/doc/riscv-bfloat16-zfbfmin.adoc>

3 Implemented replacing libcall with Zfbfmin extension instruction.

4 Reused previous testcases in:
<https://gcc.gnu.org/git/?p=gcc.git;a=commit;h=8c7cee80eb50792e57d514be1418c453ddd1073e>
gcc/ChangeLog:

	* config/riscv/iterators.md: Add mode_iterator between
	floating-point modes and BFmode.
	* config/riscv/riscv.cc (riscv_output_move): Handle BFmode move
	for zfbfmin.
	* config/riscv/riscv.md (trunc<mode>bf2): New pattern for BFmode.
	(extendbfsf2): Dotto.
	(*movhf_hardfloat): Add BFmode.
	(*mov<mode>_hardfloat): Dotto.

gcc/testsuite/ChangeLog:

	* gcc.target/riscv/zfbfmin-bf16_arithmetic.c: New test.
	* gcc.target/riscv/zfbfmin-bf16_comparison.c: New test.
	* gcc.target/riscv/zfbfmin-bf16_float_libcall_convert.c: New test.
	* gcc.target/riscv/zfbfmin-bf16_integer_libcall_convert.c: New test.

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