Throw extremely specific error if CX16 is unsupported when it shoul…

…d be.

src/processor_x86.cpp

@@ -715,10 +715,20 @@ static uint32_t sysimg_init_cb(const void *id)
     // We translate `generic` to `pentium4` or `x86-64` before sending it to LLVM
     // (see `get_llvm_target_noext`) which will be serialized into the sysimg target data.
     // Translate them back so we can actually match them.
+    bool sysimg_allows_no_cx16 = false;
     for (auto &t: sysimg) {
         if (auto nname = normalize_cpu_name(t.name)) {
             t.name = nname;
         }
+
+        // Take note to see if the sysimg explicitly allows an architecture without cx16
+        sysimg_allows_no_cx16 |= !test_nbit(t.en.features, Feature::cx16);
+    }
+    if (!sysimg_allows_no_cx16 && !test_nbit(target.en.features, Feature::cx16)) {
+        jl_error("Your CPU does not support the CX16 instruction, which is required "
+                 "by this version of Julia!  This is often due to running inside of a "
+                 "virtualized environment.  Please read "
+                 "https://docs.julialang.org/en/stable/devdocs/sysimg/ for more.");
     }
     auto match = match_sysimg_targets(sysimg, target, max_vector_size);
     // Now we've decided on which sysimg version to use.