Debug support for non-x86 architectures

#### Feature

Support debugging JITted WebAssembly code on non-x86 platforms.

#### Benefit

Currently, the debug crate only supports x86. All other platforms should be supported as well.

#### Implementation

There are a number of places that currently prevent the debug crate from supporting non-x86 platforms:

- Explicit architecture check in lib.rs:
```
match header.e_machine.get(e) {
EM_X86_64 => (),
machine => {
bail!("Unsupported ELF target machine: {:x}", machine);
}
}
```
(This should just go away, I think.)

- Explicit X86 assumptions in transform/expression.rs:
```
writer.write_op_breg(X86_64::RBP.0)?;
writer.write_sleb128(ss_offset as i64 + X86_64_STACK_OFFSET)?;
```
(This is only used for old-style back-ends and can probably go away soon.)

```
writer.write_op_breg(X86_64::RSP.0)?;
```
(This should probably use the register mapper that unwinder code also uses.)

- Various little-endian assumptions accessing ELF files and WebAssembly memory
(See https://github.com/bytecodealliance/wasmtime/pull/2854 for details.)

- Additional endian issues (not solved by the PR above) in creating DWARF expressions
Current code in transform/expression.rs simply copies portions of the incoming WebAssembly DWARF expressions directly into the native DWARF output. This is not correct in case the native architecture is big-endian. Fortunately, the byte code for many DWARF expressions is not endian-sensitive, so I can actually debug simple applications even so. But to be fully correct, those portions of DWARF bytecode that _are_ endian-sensitive will need to be handled here somehow.