Implement basic loader for Linux aarch64

This implements a basic arm64 loader for Linux, and all the currently
enabled linker tests pass.  TLS is not implemented, and functions
using it will have undefined behaviour.  Notably, the TLS test is
currently disabled on x86_64.

Much of the structure is copied from x86_64 to allow for a refactoring
of the start code between architectures.

Tested:
ninja libc_loader_tests on aarch64-linux.

Co-authored-by: Raman Tenneti <rtenneti@google.com>

Reviewed By: sivachandra

Differential Revision: https://reviews.llvm.org/D119641