ARM: zImage: make sure no GOTOFF relocs are used with .bss symbols

To be able to relocate the .bss section at run time independently from
the rest of the code, we must make sure that no GOTOFF relocations are
used with .bss symbols.  This usually means that no global variables can
be marked static unless they're also const.

To enforce this, suffice to fail the build whenever a private symbol
is allocated to .bss and list those symbols for convenience.

The user_stack and user_stack_end labels in head.S were converted into
non exported symbols to remove false positives.

Signed-off-by: Nicolas Pitre <nicolas.pitre@linaro.org>
Tested-by: Tony Lindgren <tony@atomide.com>