[HWASan] Fixed minor AArch64/GCC build error.
[HWASan] Fixed minor AArch64/GCC build error.

Fixed buildbot clang-cmake-aarch64-lld by ensuring clang-only features
are guarded by clang-only #defines.

git-svn-id: c199f293c43da69278bea8e88f92242bf3aa95f7@355840 91177308-0d34-0410-b5e6-96231b3b80d8

llvm-mirror/compiler-rt,llvm-mirror/compiler-rt,llvm-mirror/compiler-rt,llvm-mirror/compiler-rt,llvm-mirror/compiler-rt