Print backtrace upon panic

[jounathaen]

This is not an easy one, but it would make debugging hermit way more easy, if we had a RUST_BACKTRACE=1 like backtrace in the panic message.
We would probably need to analyze the stack manually for this and somehow map back the function names from the debug info to the frame pointers.

Could help e.g., in cases like:

• cargo test --test fs-test fails on main uhyve#726

[jounathaen]

This crate could be something: https://github.com/Amanieu/mini-backtrace
Unfortunately, it can't resolve the addresses by itself, but might be a first step.

[jounathaen]

I tried to implement this, but it didn't work.

Steps to reproduce:

1. Add -fPIE to libunwinds compile flags in the build script of mini-backtrace
2. compile a kernel - Note that the llvm linker has a different syntax for the linker script:

   RUSTFLAGS="-Cforce-unwind-tables -Clink-arg=-T/absolute/path/to/mini-backtrace/eh_frame.ld"  cargo build -Zbuild-std=std,panic_abort --target x86_64-unknown-hermit

3. Enjoy the General Protection Fault that occurs on let bt = Backtrace::<16>::capture();:

   [0][ERROR] General Protection (#GP) Exception: InterruptStackFrame {
      instruction_pointer: VirtAddr(
          0x8d5551,
      ),
      code_segment: SegmentSelector {
          index: 1,
          rpl: Ring0,
      },
      cpu_flags: RFlags(
          RESUME_FLAG | INTERRUPT_FLAG | AUXILIARY_CARRY_FLAG | PARITY_FLAG | 0x2,
      ),
      stack_pointer: VirtAddr(
          0x3fe588,
      ),
      stack_segment: SegmentSelector {
          index: 2,
          rpl: Ring0,
      },
   }, error 0x0