arm: make __rel_dyn_{start, end} compiler-generated

This change is only done where needed: some linker
scripts may contain relocation symbols yet remain
unchanged.

__rel_dyn_start and __rel_dyn_end each requires
its own output section; putting them in relocation
sections changes their flags and breaks relocation.

Signed-off-by: Albert ARIBAUD <albert.u.boot@aribaud.net>
Tested-by: Lubomir Popov <lpopov@mm-sol.com>
Tested-by: Jeroen Hofstee <jeroen@myspectrum.nl>
Reviewed-by: Benoît Thébaudeau <benoit.thebaudeau@advansee.com>