Successful compilation on an `aarch64` architecture

[dianpopa]

Obtain a successful cargo build --target aarch64-unknown-linux-musl.
This implies (but it is not limited to):

• Identify and label accordingly x86_64 specific code.
• Label the wrappers over the kvm ioctls in conformity with Oficial KVM API.
• Code inside the kvm_gen/src/x86_64/bindings.rs is not actually x86_64 specific. The auto generated bindings contain a common wrapper as it is autogenerated from include/uapi/linux/kvm.h and you may also find variables like KVM_PPC_PVINFO_FLAGS_EV_IDLE which are power pc related. Get rid of the x86_64 classification inside this crate.
• The default syscalls that the vmm uses are diferent on an aarch64. For example, arm does not use libc::SYS_open but libc::SYS_openat. Indeed, compare the arch/aarch64/bits/syscall.h inside the musl repo with its x86_64 equivalent (i.e arch/x86_64/bits/syscall.h).
• Not necessarily a blocker for succesful aarch64 compilation, but we are using #[cfg(any(target_arch = "x86", target_arch = "x86_64"))] mistakenly throughout our code. Firecracker would not actually compile on an x86 (see timestamp_cycles() function inside fc_util crate.
• Building with aarch64-unknown-linux-musl is not currently possible. See rust issue. See if workaround suggested works.

[dianpopa]

Above mentioned PR was merged.