arm/lib/bootm.c: keep ARM v7M in thumb mode during boot_jump_linux()

On ARM v7M, the processor will return to ARM mode when executing
a blx instruction with bit 0 of the address == 0. Always set it
to 1 to stay in thumb mode.

Tested on STM32f746-disco board

Similar commit:
f99993c1
Author: Matt Porter <mporter@konsulko.com>

Signed-off-by: Patrice Chotard <patrice.chotard@st.com>