Do not use cpuid on Miri

[AaronKutch]

When trying to run certain things on Miri, it will fail because cpuid assembly is being called somewhere. This should be configured out for the case where Miri is used. Actually, there is probably a bad default to begin with if it is using inline assembly at all, it should be falling back to entirely generic code unless a known architecture is being compiled for.

home/aaron/.rustup/toolchains/nightly-2025-08-20-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/../../stdarch/crates/core_arch/src/x86/cpuid.rs:75:9
    |
 75 | /         asm!(
 76 | |             "mov {0:r}, rbx",
 77 | |             "cpuid",
 78 | |             "xchg {0:r}, rbx",
...   |
 83 | |             options(nostack, preserves_flags),
 84 | |         );
    | |_________^ unsupported operation occurred here
    |
    = help: this is likely not a bug in the program; it indicates that the program performed an operation that Miri does not support
    = note: BACKTRACE on thread 
    = note: inside `std::arch::x86_64::__cpuid_count` at /home/aaron/.rustup/toolchains/nightly-2025-08-20-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/../../stdarch/crates/core_arch/src/x86/cpuid.rs:75:9: 84:10
    = note: inside `std::arch::x86_64::__cpuid` at /home/aaron/.rustup/toolchains/nightly-2025-08-20-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/../../stdarch/crates/core_arch/src/x86/cpuid.rs:94:5: 94:27
    = note: inside `libcrux_platform::x86::init::cpuid` at /home/aaron/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/libcrux-platform-0.0.2/src/x86.rs:123:9: 123:22
    = note: inside `libcrux_platform::x86::init` at /home/aaron/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/libcrux-platform-0.0.2/src/x86.rs:136:19: 136:27
    = note: inside `libcrux_platform::x86::supported` at /home/aaron/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/libcrux-platform-0.0.2/src/x86.rs:49:5: 49:11
    = note: inside `libcrux_platform::platform::simd256_support` at /home/aaron/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/libcrux-platform-0.0.2/src/lib.rs:92:16: 92:48
    = note: inside `libcrux_ml_kem::ind_cca::multiplexing::generate_keypair: