RISC-V: Modify _Bfloat16 to __bf16

RISC-V: Modify _Bfloat16 to __bf16

According to the description in:
<https://github.com/riscv-non-isa/riscv-elf-psabi-doc/pull/367>,
the type representation symbol of BF16 has been corrected.

Kito Cheng pointed out relevant information in the email:
<https://gcc.gnu.org/pipermail/gcc-patches/2024-May/651850.html>

gcc/ChangeLog:

	* config/riscv/riscv-builtins.cc (riscv_init_builtin_types):
	Modify _Bfloat16 to __bf16.
	* config/riscv/riscv.cc (riscv_mangle_type): Ditto.

gcc/testsuite/ChangeLog:

	* gcc.target/riscv/_Bfloat16-nanboxing.c: Move to...
	* gcc.target/riscv/__bf16-nanboxing.c: ...here.
	* gcc.target/riscv/bf16_arithmetic.c: Modify _Bfloat16 to __bf16.
	* gcc.target/riscv/bf16_call.c: Ditto.
	* gcc.target/riscv/bf16_comparison.c: Ditto.
	* gcc.target/riscv/bf16_float_libcall_convert.c: Ditto.
	* gcc.target/riscv/bf16_integer_libcall_convert.c: Ditto.