[POWERPC] Fix COMMON symbol warnings

We get the following warnings in various ARCH=powerpc builds:

WARNING: "ee_restarts" [arch/powerpc/kernel/built-in] is COMMON symbol
WARNING: "fee_restarts" [arch/powerpc/kernel/built-in] is COMMON symbol
WARNING: "htab_hash_searches" [arch/powerpc/mm/built-in] is COMMON symbol
WARNING: "next_slot" [arch/powerpc/mm/built-in] is COMMON symbol
WARNING: "mmu_hash_lock" [arch/powerpc/mm/built-in] is COMMON symbol
WARNING: "primary_pteg_full" [arch/powerpc/mm/built-in] is COMMON symbol
WARNING: "global_dbcr0" [arch/powerpc/kernel/built-in] is COMMON symbol

Switch to moving local symbols (except mmu_hash_lock which is global) and
space directive instead.

Signed-off-by: Kumar Gala <galak@kernel.crashing.org>