Current ARM assembler helper for the 'return to caller' pseudo-instruction
turns 'ret lr' into 'mov pc, lr' for ARMv5TE. This causes the core to remain
in its current ARM state even when the routine doing the 'ret' was called
from Thumb-1 state, triggering an undefined instruction exception.

This causes early run-time failures in all boards compiled using the Thumb-1
instruction set (for instance the Open-RD family).

ARMv5TE supports 'bx lr' which properly implements interworking and thus
correctly returns to Thumb-1 state from ARM state.

This change makes 'ret lr' turn into 'bx lr' for ARMv5TE.

Signed-off-by: Albert ARIBAUD <albert.u.boot@aribaud.net>