Selectable driver from main executable for testing

[rdrpenguin04]

The current state of lcld prohibits testing. Cargo can't copy files as a post-build task, and the main executable can't be run as a linker. For testing purposes, the linker should be able to be run as such:

• lcld --mode win <remaining arguments passed to lc-link driver>
• lcld --mode mac <remaining arguments passed to ld64.lc driver>
• lcld --mode posix <remaining arguments passed to ld.lc driver>