[libc++] Fix simd_unary.pass.cpp with AppleClang

When using AppleClang the `clang` feature flag is not set, but the
compiler supports `-flax-vector-conversions=integer`. This adds another
`ADDITIONAL_COMPILE_FLAGS` for AppleClang to fix the CI.

Author: philnik777

libcxx/test/std/experimental/simd/simd.class/simd_unary.pass.cpp

@@ -14,6 +14,7 @@
 
 // FIXME: This should work with -flax-vector-conversions=none
 // ADDITIONAL_COMPILE_FLAGS(clang): -flax-vector-conversions=integer
+// ADDITIONAL_COMPILE_FLAGS(apple-clang): -flax-vector-conversions=integer
 
 // <experimental/simd>
 //