RISC-V LLVM feature +unaligned-scalar-mem not recognized by rustc

[korran]

Example

// Compile with rustc --target riscv32imc-unknown-none-elf -C target-feature=+unaligned-scalar-mem --emit=obj
#![no_std]
#![no_main]

use core::panic::PanicInfo;

#[panic_handler]
fn panic(_info: &PanicInfo) -> ! {
    loop {}
}

Current output

warning: unknown feature specified for `-Ctarget-feature`: `unaligned-scalar-mem`
  |
  = note: it is still passed through to the codegen backend
  = help: consider filing a feature request

warning: 1 warning emitted

Desired output

No warning

Background / Rationale

Some RISC-V cores support unaligned access to memory without trapping. On such cores, the compiler could significantly improve code-size and performance when using functions like core::ptr::read_unaligned by emitting a single load or store instruction with an unaligned address, rather than a long sequence of byte-sized load/store/bitmanip instructions.

I need to use the unaligned-scalar-mem target feature to improve code-size and performance. When I set the flag -C target-feature=+unaligned-scalar-mem, LLVM successfully recognizes the target feature and generates the desired machine code, but rustc prints out the warning above, and I can't figure out how to disable it.