decompressors: group XZ_DEC_* symbols under an if XZ_BCJ / endif

Group all architecture-specific BCJ filter configuration symbols under an
if XZ_BCJ / endif statement.

Signed-off-by: Florian Fainelli <florian@openwrt.org>
Acked-by: Lasse Collin <lasse.collin@tukaani.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>