Adds native support for Fiber context switching on RISC-V

[denizzzka]

This code is based on mine earlier implementation for rv32, so both platforms should be supported: rv32 and rv64 with and without compressed instructions and with and without HardFloat.

For now it is tested only on rv64 with HardFloat (QEMU, Ubuntu 24.10, ldc2 from ~master)

Implemented using separate file because I see potential for growth of this assembly code (I want to implement embedded ISA subset and maybe something vendor-specific), and also to put an end to vicious scaring me practice of putting assembly code for completely different platforms into one huge .S file

Path to the new asm file is the same as in the current DMD ~master for core.thread.fiber package

[denizzzka]

@wszqkzqk Hi! Maybe you are also interested in this?

[kinke]

For posterity: I've 'pushed' for this upstream in dlang/dmd#20964, but turns out GDC doesn't use that upstream core/thread/fiber/switch_context_asm.S file anymore, but has its own set of asm files apparently. And the .S file only handles archs that DMD cannot target, so it isn't really used by DMD either. So this separate file here for RISC-V and LDC alone seems okay to me for now.

So thx Dennis for bearing with me.