Fix loader on ARM platforms without SMC available/starting at EL3

See Mattermost discussion https://mattermost.trustworthy.systems/sel4-external/pl/f7ibsmm6zfrzjgb1rhfu4ewu1a. The fix is fairly trivial for when starting at EL3 but it's less obvious how to fix SMC not being available, other than with a config option. The only thing I can think of is maybe just skipping SMC calls if we are entered at EL3, but I don't know how reliable that is.