Make aeskeygenassist imm8 argument i32, not u8

Intel documentation specifies it as an "8-bit round constant", but then
goes on to give it a type "const int", which translates to i32 in Rust.
The test that verifies the Rust signatures against Intel documentation
failed on this.

For now we will replicate the C API verbatim. Even when Rust could have
a more accurate type signature that makes passing values more than 8
bits impossible, rather than silently mapping out-of-range values to
255.

Author: ruuda

coresimd/src/x86/i686/aes.rs

@@ -16,7 +16,7 @@ extern "C" {
     #[link_name = "llvm.x86.aesni.aesimc"]
     fn aesimc(a: __m128i) -> __m128i;
     #[link_name = "llvm.x86.aesni.aeskeygenassist"]
-    fn aeskeygenassist(a: __m128i, imm8: u8) -> __m128i;
+    fn aeskeygenassist(a: __m128i, imm8: i32) -> __m128i;
 }
 
 /// Perform one round of an AES decryption flow on data (state) in `a`.
@@ -62,11 +62,12 @@ pub unsafe fn _mm_aesimc_si128(a: __m128i) -> __m128i {
 /// Assist in expanding the AES cipher key.
 ///
 /// Assist in expanding the AES cipher key by computing steps towards generating
-/// a round key for encryption cipher using data from `a` and an 8-bit round constant.
+/// a round key for encryption cipher using data from `a` and an 8-bit round
+/// constant `imm8`.
 #[inline]
 #[target_feature(enable = "aes")]
 #[cfg_attr(test, assert_instr(aeskeygenassist, imm8 = 0))]
-pub unsafe fn _mm_aeskeygenassist_si128(a: __m128i, imm8: u8) -> __m128i {
+pub unsafe fn _mm_aeskeygenassist_si128(a: __m128i, imm8: i32) -> __m128i {
     macro_rules! call {
         ($imm8:expr) => (aeskeygenassist(a, $imm8))
     }