Makfile: Add target flags for `clang` with `AArch64`

Makfile: Add target flags for `clang` with `AArch64`

This commit adds target flags needed for cross-compiling with `clang`
for`AArch64`.
They are guarded by a condition which checks if these flags
are really needed (the compiler is `clang` and the architecture is
`AArch64`).

Signed-off-by: Maria Sfiraiala <maria.sfiraiala@gmail.com>
Reviewed-by: Stefan Jumarea <stefanjumarea02@gmail.com>
Approved-by: Michalis Pappas <michalis@unikraft.io>
Tested-by: Unikraft CI <monkey@unikraft.io>
GitHub-Closes: #685