[Frontend] Allow OpenMP offloading to aarch64

Summary:
D30644 added OpenMP offloading to AArch64 targets, then D32035 changed the
frontend to throw an error when offloading is requested for an unsupported
target architecture. However the latter did not include AArch64 in the list
of supported architectures, causing the following unit tests to fail:

    libomptarget :: api/omp_get_num_devices.c
    libomptarget :: mapping/pr38704.c
    libomptarget :: offloading/offloading_success.c
    libomptarget :: offloading/offloading_success.cpp

Reviewers: pawosm01, gtbercea, jdoerfert, ABataev

Subscribers: kristof.beyls, guansong, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D70804