Auto merge of #42500 - hsivonen:neon, r=japaric

Change `llvm.neon.*` to `llvm.arm.neon.*` in the mapping for platform intrinsics

This avoids linker errors when using platform intrinsics on 32-bit ARM with
NEON.

Fixes rust-lang-nursery/simd#10.

I don't have an explanation why the old code might have worked when the `simd` crate was written, since the new LLVM intrinsic naming seems to have existed as early as 2012. Maybe LLVM accepted two naming schemes for a few years and dropped the `llvm.neon.*` version recently? I don't know.

Also, I don't know how to write a unit test for this.

src/etc/platform-intrinsics/arm.json

@@ -1,7 +1,7 @@
 {
     "platform": "arm_v",
     "intrinsic_prefix": "",
-    "llvm_prefix": "llvm.neon.v",
+    "llvm_prefix": "llvm.arm.neon.v",
     "number_info": {
         "signed": {
             "kind": "s",