crash_dump: Make elfcorehdr address/size symbols always visible

crash_dump: Make elfcorehdr address/size symbols always visible

Make the forward declarations of elfcorehdr_addr and elfcorehdr_size,
and the definitions of ELFCORE_ADDR_MAX and ELFCORE_ADDR_ERR always
available, like is done for phys_initrd_start and phys_initrd_size.
Code referring to these symbols can then just check for
IS_ENABLED(CONFIG_CRASH_DUMP), instead of requiring conditional
compilation using an #ifdef, thus preparing to increase compile
coverage.

Suggested-by: Rob Herring <robh+dt@kernel.org>
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/ba965ca613c0cc82c1ec2fe353ee34fb13b36474.1628670468.git.geert+renesas@glider.be