Set a "kernbase" symbol in 32-bit arm locore.S and use it with ldscript.arm.

This brings arm into line with how every other arch does it. For some
reason, only arm lacked a definition of a symbol named kernbase in its
locore.S file(s) for use in its ldscript.arm file. Needlessly different
means harder to maintain.

Using a common symbol name also eases work in progress on a script to help
generate arm and arm64 kernels packaged in various ways (like with a header
blob needed for a bootloader prepended to the kernel file).