loader: Use TERM_EMU for arm and arm64

Even if it didn't behave well previously this is fixed.
Tested on: OrangePi One (armv7 u-boot) (serial only and serial + HDMI)
Tested on: Pine64-LTS (aarch64 u-boot) (serial only and serial + HDMI)
Tested on: Honeycomb (aarch64 EDK2) (serial only)

Reviewed by: imp
Differential Revision: https://reviews.freebsd.org/D28153