Accessing 8bit and 16bit constants from ROM rather than RAM comes with a
performance cost, as these loads go through the load/store exception
vector. Any performance critical constants can be forced back into RAM
as RAM_CONST_ATTR.
The entry point has changed from call_user_start() to user_start_trampoline()
in order for the exception handler to be installed early enough.
b0f9788a6d