ARM: Fix __bss_start and __bss_end in linker scripts

Commit 3ebd1cbc introduced compiler-generated __bss_start
and __bss_end__ and commit c23561e7 rewrote all __bss_end__
as __bss_end. Their merge caused silent and harmless but
potentially bug-inducing clashes between compiler- and linker-
generated __bss_end symbols.

Make __bss_end and __bss_start compiler-only, and create
__bss_base and __bss_limit for linker-only use.

Signed-off-by: Albert ARIBAUD <albert.u.boot@aribaud.net>
Reported-by: Benoît Thébaudeau <benoit.thebaudeau@advansee.com>