unix: have Makefile detect MIPS and x64 options

If no ARCH variable is passed, then the Makefile will attempt to detect platform-specific options (MIPS and non-x86-64.

Seems to work for native and cross-compilation (ARCH must be passed for the latter).
Better to simply use: `ARCH ?= $(UNAME_M)` instead of whole ifeq/override/endif.