Remove gcc special case in favor of __builtin_convertvector

Noticed a warning when compiling with gcc locally and this seems like
a sensible resoltuion.

Change-Id: I23d15373b4fa3ace60559d6350e0a38a5eeb582a
Reviewed-on: https://skia-review.googlesource.com/c/skcms/+/1038877
Reviewed-by: Daniel Dilan <danieldilan@google.com>
Commit-Queue: Kaylee Lubick <kjlubick@google.com>
Auto-Submit: Kaylee Lubick <kjlubick@google.com>

src/Transform_inl.h

diff --git a/src/Transform_inl.h b/src/Transform_inl.h
index 99faf25..162620d 100644
--- a/src/Transform_inl.h
+++ b/src/Transform_inl.h
@@ -111,14 +111,9 @@
 SI D cast(const S& v) {
 #if N == 1
     return (D)v;
-#elif defined(__clang__)
-    return __builtin_convertvector(v, D);
 #else
-    D d;
-    for (int i = 0; i < N; i++) {
-        d[i] = v[i];
-    }
-    return d;
+    return __builtin_convertvector(v, D);
+
 #endif
 }