sparc: prepare kernel/ for unification

o sparc32 files with identical names to sparc64 renamed to <name>_32.S
o introduced a few Kconfig helpers to simplify Makefile logic
o refactored Makefile to prepare for unification
  - use obj-$(CONFIG_SPARC32) for sparc32 specific files
  - use <name>_$(BITS) for files where sparc64 has a _64 variant
  - sparc64 directly include a few files where sparc32 builds them,
    refer to these files directly (no BITS)
  - sneaked in -Werror as used by sparc64
o modified sparc/Makefile to use the new names for head/init_task

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: David S. Miller <davem@davemloft.net>