Add a `sycall_fnname` function (in user-space)

[vadorovsky]

Kernel symbols representing syscalls have architecture-specific prefixes. For example, for x86_64 it's __x64 (so sys_execve syscall is evaluated to __x64_sys_execve).

BCC provides a function for resolving syscall names (get_sycall_fnname). It would be nice to have it in Aya as well (with a more Rusty name like syscall_fname or something better). Currently, developers using Aya need to resolve those name manually per architecture, which adds boilerplate code to their projects.

[alessandrod]

We should definitely not have anything called get_sycall_fnname in our API, but I'm open to a better name 😋

[alessandrod]

I think we should have something like fn syscall_prefix() -> &str { ... } instead of syscall_fname. syscall_fname("lol") succeeds but isn't really a syscall. Then people would do let syscall_name = format!("{}execve", syscall_prefix());

[vadorovsky]

Done in #625