Temporary fix: change the feature gate of VEX variants

These were causing compilation failures while stabilizing avx512, will be FCP'd soon, together with the rest of AVX512

Author: sayantn

crates/core_arch/src/lib.rs

@@ -23,6 +23,7 @@
     riscv_target_feature,
     arm_target_feature,
     avx512_target_feature,
+    stdarch_x86_avx512,
     mips_target_feature,
     powerpc_target_feature,
     s390x_target_feature,

crates/std_detect/src/detect/arch/x86.rs

@@ -197,15 +197,15 @@ features! {
     /// AVX-512 P2INTERSECT
     @FEATURE: #[stable(feature = "simd_x86", since = "1.27.0")] avx512fp16: "avx512fp16";
     /// AVX-512 FP16 (FLOAT16 instructions)
-    @FEATURE: #[unstable(feature = "avx512_target_feature", issue = "44839")] avxifma: "avxifma";
+    @FEATURE: #[unstable(feature = "stdarch_x86_avx512", issue = "44839")] avxifma: "avxifma";
     /// AVX-IFMA (Integer Fused Multiply Add)
-    @FEATURE: #[unstable(feature = "avx512_target_feature", issue = "44839")] avxneconvert: "avxneconvert";
+    @FEATURE: #[unstable(feature = "stdarch_x86_avx512", issue = "44839")] avxneconvert: "avxneconvert";
     /// AVX-NE-CONVERT (Exceptionless Convert)
-    @FEATURE: #[unstable(feature = "avx512_target_feature", issue = "44839")] avxvnni: "avxvnni";
+    @FEATURE: #[unstable(feature = "stdarch_x86_avx512", issue = "44839")] avxvnni: "avxvnni";
     /// AVX-VNNI (Vector Neural Network Instructions)
-    @FEATURE: #[unstable(feature = "avx512_target_feature", issue = "44839")] avxvnniint16: "avxvnniint16";
+    @FEATURE: #[unstable(feature = "stdarch_x86_avx512", issue = "44839")] avxvnniint16: "avxvnniint16";
     /// AVX-VNNI_INT8 (VNNI with 16-bit Integers)
-    @FEATURE: #[unstable(feature = "avx512_target_feature", issue = "44839")] avxvnniint8: "avxvnniint8";
+    @FEATURE: #[unstable(feature = "stdarch_x86_avx512", issue = "44839")] avxvnniint8: "avxvnniint8";
     /// AVX-VNNI_INT16 (VNNI with 8-bit integers)
     @FEATURE: #[unstable(feature = "x86_amx_intrinsics", issue = "126622")] amx_tile: "amx-tile";
     /// AMX (Advanced Matrix Extensions) - Tile load/store

crates/std_detect/tests/x86-specific.rs

@@ -3,6 +3,7 @@
 #![feature(
     stdarch_internal,
     avx512_target_feature,
+    stdarch_x86_avx512,
     sha512_sm_x86,
     x86_amx_intrinsics,
     xop_target_feature,