kboot: aarch64 support

Add support for aarch64. exec.c and ldscript are copied from the EFI
version with #ifdefs for the differences. Once complete, I'll refactor
them. host_syscall.S implements a generic system call. tramp.S is a
first attempt to create a tramoline that we can use to jump to the
aarch64 kernel. Add aarch64-specific startup and stat files as well.
exec.c tweaked slightly to avoid bringing in bi_load(), which will come
in later. Includes tweaks to stat due to name differences between names
on different Linux architectures.

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