Backtrace support

[arihant2math]

Mostly glued together from the original redox implementation.

Passing --demangle will cause qemu to scan for FP and PC and will running symbol demangling on the host machine.

stdin/stdout is somewhat broken in --demangle, however it works.

Example:

/ # times
[   33.33618002] moss: Kernel panicked at src/arch/arm64/exceptions/syscall.rs:818:14: Unhandled syscall 0x99, PC: 0x7000000681a0
[   33.33619426] backtrace:   FP ffffb80000017980: PC ffff8000000acdf4
        => __rustc::rust_begin_unwind at /Users/anaren/Documents/oss/moss/src/main.rs:80
[   33.33619598] backtrace:   FP ffffb80000017ab0: PC ffff800000120b04
        => core::ptr::align_offset::<u8> at /rustc/64a965e9013a9d14e83c4d370af26f6be6bf96fb/library/core/src/ptr/mod.rs:2308
[   33.33619682] backtrace:   FP ffffb80000017af0: PC ffff8000000720f0
        => moss::arch::arm64::exceptions::syscall::handle_syscall::{closure#0} at /Users/anaren/Documents/oss/moss/src/arch/arm64/exceptions/syscall.rs:120
[   33.33619750] backtrace:   FP ffffb80000017b40: PC ffff8000000cadf4
        => moss::sched::uspc_ret::dispatch_userspace_task at /Users/anaren/Documents/oss/moss/src/sched/uspc_ret.rs:148
[   33.33619832] backtrace:   FP ffffb80000017de0: PC ffff8000000fb5dc
        => el0_sync at /Users/anaren/Documents/oss/moss/src/arch/arm64/exceptions/mod.rs:185
[   33.33619902] backtrace:   FP ffffb80000017ea0: PC ffffe000000009a0
        => ?? ??:0
[   33.33620010] backtrace:   00007fffffffff10: INVALID FRAME