[clang] Improve hermeticity of clang header tests.

At the moment the below header tests fail with the multilib error in LLVM Embedded Toolchain for Arm because there is no corresponding aarch64 big endian library variant  exist. Specifying --sysroot to its own testing directory clang/test/Headers/Inputs (which does not have any dependency library) prevents these header tests  from being located in standard library directories.

 1. clang/test/Headers/arm-neon-header.c
 2. clang/test/Headers/arm-fp16-header.c

Reviewed By: michaelplatings

Differential Revision: https://reviews.llvm.org/D156427