htool: add macos build

[mfield4]

This lets htool build and run on macOS with Bazel, using the USB transport. It also adds a macOS job to CI. Linux only transports are not ported, and choosing one on mac exits with a printed warning and a non-zero exit code.

Tested on real hardware from an Apple Silicon Mac:

$ ./bazel-bin/examples/htool --usb_loc 0-1.3 hello
output: 0x01020304

$ ./bazel-bin/examples/htool --usb_loc 0-1.3 show chipinfo
Chip Info:
...

Known limitations / follow-ups

• No -Werror on macOS yet. Format strings like %lx for uint64_t/libhoth_error warn on macOS, where those are unsigned long long. Switching to PRIx64 would fix it. libusb's upstream darwin_usb.c also has unused-variable warnings.
• The fully_static_link_target feature on htool has no effect on macOS. macOS doesn't support fully static binaries, so the result links against system libraries and frameworks dynamically.
• The linker prints ld: warning: ignoring duplicate libraries: '-lobjc'. It's harmless, because the macOS toolchain already links libobjc.