[M68k][Driver] Rename target features and macros test files

test/Driver/m68k-features.cpp -> test/Driver/m68k-macros.cpp
test/Driver/m68k-fixed-register.c -> test/Driver/m68k-features.cpp

The original m68k-features.cpp should really be called m68k-macros.cpp
since it's testing built-in macro definitions rather than sub-target
features. Which are part of what m68k-fixed-register.c was previously
doing. NFC.