make : support overriding CFLAGS/CXXFLAGS/CPPFLAGS/LDFLAGS (#2886)
* make : remove unused -DGGML_BIG_ENDIAN
* make : put preprocessor stuff in CPPFLAGS
* make : pass Raspberry Pi arch flags to g++ as well
* make : support overriding CFLAGS/CXXFLAGS/CPPFLAGS/LDFLAGS
* make : fix inverted conditional