polyval: add avx2 to runtime CPU feature check

The `cpufeatures::new!` macro in `avx2.rs` only checks for
`pclmulqdq`, but the functions it guards (`expand_key`,
`proc_block`, `proc_par_blocks`) are annotated with
`#[target_feature(enable = "avx2", enable = "pclmulqdq")]`.

On CPUs that support PCLMULQDQ but lack AVX2 (e.g. Intel Pentium
Gold, Celeron, some Atom processors), the runtime check passes,
VEX-encoded instructions are executed, and the process crashes
with SIGILL.

Adding `"avx2"` to the feature check ensures the intrinsics path
is only used when both features are available. CPUs without AVX2
correctly fall back to the software implementation.

Fixes #315

Author: wittjeff

polyval/src/backend/intrinsics/avx2.rs

@@ -31,7 +31,7 @@ use core::arch::x86_64::*;
 /// P1 polynomial: x^63 + x^62 + x^57 = 0xC200000000000000
 const P1: u64 = 0xC200000000000000;
 
-cpufeatures::new!(clmul, "pclmulqdq");
+cpufeatures::new!(clmul, "pclmulqdq", "avx2");
 pub(crate) use clmul::InitToken;
 
 /// Byte array which is the inner type of `FieldElement`