Fixed the issue with the incorrect instruction set architecture of the chip used in CH32V307.

[Snow-in-Dublin]

According to the official manual, the instruction set architecture of QingkeV4 is RV32imac rather than RV32imfc（Is this a genuine instruction set architecture?）, and when testing on the evaluation board, it must be modified to RV32imac to compile.

[romainreignier]

See this commit: https://github.com/ch32-rs/ch32-hal/commit/14e500744ef4aef2a07d8592e2760be816e473fe

What is your build issue?

[Snow-in-Dublin]

I didn't encounter any compilation issues, I just felt that the instruction set wasn't quite right.

[chmousset]

@Snow-in-Dublin CH32V307 is Qingke V4F, which is RV32IMACF The original code looks right to me.

Did you witness illegal instruction or other issues when using floating-point ?

[Snow-in-Dublin]

@chmousset My mistake, I misread it as QingkeV4C.