Fix target directory when target argument is a path

[parasyte]

I maintain cargo-n64 which uses a target JSON file (opposed to a target triple). When building a project with cargo-asm using a path as the target, it is unable to find the target directory:

[ERROR][cargo_asm::target][/Users/parasyte/.cargo/registry/src/github.com-1ecc6299db9ec823/cargo-asm-0.1.16/src/target.rs:103]: The target directory for the build does not exist: /var/folders/f0/yjjsr3hj2418r3lpmkk9gnt40000gn/T/n64-build/mips-nintendo64-none.json/release/deps
thread 'main' panicked at 'failed to iterate over the directory: /var/folders/f0/yjjsr3hj2418r3lpmkk9gnt40000gn/T/n64-build/mips-nintendo64-none.json/release/deps: Error { depth: 0, inner: Io { path: Some("/var/folders/f0/yjjsr3hj2418r3lpmkk9gnt40000gn/T/n64-build/mips-nintendo64-none.json/release/deps"), err: Os { code: 20, kind: Other, message: "Not a directory" } } }', /Users/parasyte/.cargo/registry/src/github.com-1ecc6299db9ec823/cargo-asm-0.1.16/src/build.rs:122:17
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

This PR fixes the bug by detecting whether the target argument is a path (and points to a file that exists), then parses it down to the expected triple.