[ARM] Add missing target for Arm neon test case.

This is a follow-up from https://reviews.llvm.org/D61717. Where Richard
described the issue with compiling arm_neon.h under
-flax-vector-conversions=none. It looks like the example reproducer does
actually work but what was missing was a test entry for that target.

Differential Revision: https://reviews.llvm.org/D88546

Author: rs-arm

clang/test/Headers/arm-neon-header.c

@@ -22,5 +22,6 @@
 
 // RUN: %clang -fsyntax-only -Wall -Werror -ffreestanding --target=aarch64-none-eabi -march=armv8.2-a+fp16fml+crypto+dotprod -std=c11 -xc -flax-vector-conversions=none %s
 // RUN: %clang -fsyntax-only -Wall -Werror -ffreestanding --target=aarch64_be-none-eabi -march=armv8.2-a+fp16fml+crypto+dotprod -std=c11 -xc -flax-vector-conversions=none %s
+// RUN: %clang -fsyntax-only -Wall -Werror -ffreestanding --target=arm64-linux-gnu -arch +neon -std=c11 -xc -flax-vector-conversions=none %s
 
 #include <arm_neon.h>