build: Remove WASI_SNAPSHOT and THREAD_MODEL and make TARGET_TRIPLE the single source of truth

[catamorphism]

See @alexcrichton's suggestion at #599 (comment) . Currently the top-level and test Makefiles allow WASI_SNAPSHOT, THREAD_MODEL, and TARGET_TRIPLE to be set from the outside, but TARGET_TRIPLE is ignored if WASI_SNAPSHOT or THREAD_MODEL are set. Alex suggested removing WASI_SNAPSHOT and THREAD_MODEL altogether and just setting TARGET_TRIPLE to what's provided on the command-line (keeping the current default).