fix: AMD64 cpus not detected as X86_64 on FreeBSD

Linux and Rust both seems to normalize amd64 CPUs to x86_64 for AMD CPUs, but FreeBSD will return amd64 for both uname -m and sysctl hw.machine. While rustup treats both x86_64 and amd64, rustup uses the same triple as cc -dumpmachine.

E.g.:

% uname -m
amd64

% sysctl hw.machine
hw.machine: amd64

% cc -dumpmachine
x86_64-unknown-freebsd14.2

% rustup show | grep host
Default host: x86_64-unknown-freebsd

Here is the output of the example guess-host-triple binary on this host, from current master and from this branch:

% RUST_LOG=debug cargo run --example guess-host-triple
[ snip warnings ]
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.03s
     Running `target/debug/examples/guess-host-triple`
 WARN 2025-06-30T09:09:50Z: guess_host_triple: uname returned unrecognised machine "amd64"
 WARN 2025-06-30T09:09:50Z: guess_host_triple: Could not guess triple for (Unknown, Little, FreeBsd)
Could not determine host triple

% git co fix/amd64-misidentifed
Switched to branch 'fix/amd64-misidentifed'

% RUST_LOG=debug cargo run --example guess-host-triple
[ snip lint warnings ]
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.41s
     Running `target/debug/examples/guess-host-triple`
x86_64-unknown-freebsd

I also fixed a couple very basic linter warnings, but didn't get very ambitious, as I don't have all the archs covered.