sparc: unify sparc32/sparc64 archhelp

Currently, entering
$ make ARCH=sparc32 help
prints the archhelp text for sparc64.
["sparc32" is documented (Documentation/kbuild/kbuild.rst)
to be a recognized alias for 32-bit sparc.]

Instead of handling ARCH=sparc or ARCH=sparc32 or ARCH=sparc64,
just unify all SPARC archhelp text in one place.

Fixes: 5e538790 ("sparc,sparc64: unify Makefile")
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Suggested-by: Masahiro Yamada <masahiroy@kernel.org>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>