32-bit ARM linux_raw backend has MSRV 1.82, instead of MSRV 1.63, due to use of `asm_const`

[briansmith]

Building with the MSRV with the "linux_raw" backend enabled, like this:

$ rustup +1.63 target add armv7-unknown-linux-gnueabihf
$ RUSTFLAGS="--cfg getrandom_backend=\"linux_raw\"" cargo +1.63 check --target=armv7-unknown-linux-gnueabihf

results in an error:

error[E0658]: const operands for inline assembly are unstable
  --> src/backends/linux_raw.rs:29:17
   |
29 |                 nr = const __NR_getrandom,
   |                 ^^^^^^^^^^^^^^^^^^^^^^^^^
   |
   = note: see issue #93332 <https://github.com/rust-lang/rust/issues/93332> for more information

For more information about this error, try `rustc --explain E0658`.

asm_const wasn't stabilized until Rust 1.82 (see rust-lang/rust#128570).

[newpavlov]

We could inline the constant, but we do not guarantee that all backends have the same MSRV as the crate (e.g. like with wasi_p1/p2 or efi_rng), so I am fine with either option (leaving as-is or inlining).