efl: add compiler flags
- -fvisibility=hidden will hide symbols not marked with EAPI in the
final binary (so/executable).
- -ffunction-sections and -fdata-sections will split those into
independent elf sections, then we can -Wl,--gc-sections for those that are unused. During development --Wl,--print-gc-sections will warn us of functions that are left unused and were collected.