[libc] Add a simple linux aarch64 config.

Summary:
With this change, "ninja check-libc" on linux/aarch64 succeeds.

However, all entrypoints with machine dependent implementations
have been skipped. A good number of these skipped entrypoints can
be enabled once we have aarch64 syscall support available.

Reviewers: abrachet, asteinhauser

Differential Revision: https://reviews.llvm.org/D81533