xen/arm32: head: Move earlyprintk messages to .rodata.str

xen/arm32: head: Move earlyprintk messages to .rodata.str

At the moment, the strings are in text right after each use because
the instruction 'adr' has specific requirement on the location
and the compiler will forbid cross section label.

The macro 'adr_l' was recently reworked so the caller doesn't need
to know whether the MMU is on. This makes it easier to use where
instructions can be run in both context.

This also means that the strings don't need to be part of .text
anymore. So move them to .rodata.str.

Signed-off-by: Julien Grall <jgrall@amazon.com>
Reviewed-by: Jiamei Xie <jiamei.xie@arm.com>
Reviewed-by: Bertrand Marquis <bertrand.marquis@arm.com>
Tested-by: Bertrand Marquis <bertrand.marquis@arm.com>
Reviewed-by: Wei Chen <Wei.Chen@arm.com>