remove all references to .dynsym

Discard all .dynsym sections from linker scripts
Remove all __dynsym_start definitions from linker scripts
Remove all __dynsym_start references from the codebase

Note: this touches include/asm-generic/sections.h, which
is not ARM-specific, but actual uses of __dynsym_start
are only in ARM, so this patch can safely go through
the ARM repository.

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>