Auto-detection of RV32E

[lrettig]

If we're ever going to support both riscv32e and riscv32i, it would be helpful to remove the hardcoded rv32e feature flag added in #15 and make this detection automatic instead. See #1 (comment).

I don't think it's possible to do this using plain compiler directives. They're not granular enough to distinguish between the two; both show up as target_arch = riscv32. However auto-detection could be done in a build script based on the target:

build.rs:

use std::env;

fn main() {
    // Example: Check an environment variable to determine if we're targeting `riscv32e`
    let target = env::var("TARGET").unwrap_or_default();
    if target.contains("riscv32e") {
        println!("cargo:rustc-cfg=feature=\"riscv32e\"");
    }
}

inline:

#![cfg_attr(feature = "riscv32e", allow(dead_code))]

#[cfg(feature = "riscv32e")]
fn riscv32e_specific_function() {
    // Implementation for riscv32e
}