feat: support save and restore fp regs for loongarch

[MF-B]

Description

Implement floating point support for loongarch

Implementation Details

• Added a feature flag "fp_simd" in polyhal crate to conditionally enable floating point support
• Implemented two functions in polyhal::kcontext module:
  • save_fp_regs: Saves floating point register state to a KContext
  • restore_fp_regs: Restores floating point register state from a KContext

How to test

I added a test function in the example crate:

use polyhal::kcontext::KContext;
use polyhal::kcontext::save_fp_regs;
use polyhal::kcontext::restore_fp_regs;

pub fn test_fp_registers() {
    log::info!("Testing FP register save/restore...");
    
    // 创建两个上下文
    let mut context1 = KContext::blank();
    let mut context2 = KContext::blank();
    
    // 定义要使用的浮点常量
    let pi = 3.14159_f64;
    let e = 2.71828_f64;
    let sqrt2 = 1.41421_f64;
    let sqrt3 = 1.73205_f64;
    
    // 初始化一些浮点寄存器值
    unsafe {
        // 使用内联汇编设置一些浮点寄存器的值
        core::arch::asm!(
            "
            fld.d   $f1, {pi_ptr}, 0
            fld.d   $f2, {e_ptr}, 0
            ",
            pi_ptr = in(reg) &pi,
            e_ptr = in(reg) &e,
        );
        
        // 保存当前浮点寄存器状态到context1
        save_fp_regs(&mut context1);
        
        // 改变浮点寄存器的值
        core::arch::asm!(
            "
            fld.d   $f1, {sqrt2_ptr}, 0
            fld.d   $f2, {sqrt3_ptr}, 0
            ",
            sqrt2_ptr = in(reg) &sqrt2,
            sqrt3_ptr = in(reg) &sqrt3,
        );
        
        // 保存更改后的浮点寄存器状态到context2
        save_fp_regs(&mut context2);
        
        // 恢复context1的浮点寄存器状态
        restore_fp_regs(&context1);
        
        // 验证是否成功恢复
        let f1_value: f64 = 0.0;
        let f2_value: f64 = 0.0;
        core::arch::asm!(
            "
            fst.d   $f1, {f1_ptr}, 0
            fst.d   $f2, {f2_ptr}, 0
            ",
            f1_ptr = in(reg) &f1_value,
            f2_ptr = in(reg) &f2_value,
        );
        
        log::info!("After restore, f1 value = {}, f2 value = {}", f1_value, f2_value);
        // 理论上f1_value应该接近3.14159，f2_value应该接近2.71828
        
        if (f1_value - 3.14159).abs() < 0.0001 && (f2_value - 2.71828).abs() < 0.0001 {
            log::info!("FP register save/restore test passed!");
        } else {
            log::error!("FP register save/restore test failed! f1={}, f2={}", f1_value, f2_value);
        }
    }
}

Test Result

With restore_fp_regs disabled:

With both two enabled:

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[MF-B]

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