riscv-rt: Implement FPU initialization

[mini-ninja-64]

This PR adds FPU initialization when compiling with the f or d extensions, for the implementation of this I have updated the parse_target function from build.rs to support additional extensions, this is required until this crate is updated to rust 1.75 at which point we can use target_feature.

I added a little macro to help with zeroing registers, if it is preferable though happy to inline the zeroing of registers

I am new to rust & risc-v in general so apologies if there is any obvious mistakes that have been made, your feedback is greatly appreciated.

closes: #157

[romancardenas]

Thanks! I'll review your PR ASAP

[romancardenas]

Looks good to me!! I'll let a couple of days in case anyone from @rust-embedded/risc-v finds any issue and then I'll add it to the merge queue

[mini-ninja-64]

wooo, thank you so much ☺️