xzcat: more cleanup and some shrinking.

-drop __always_inline (c. 1.5k of binary size),
-remove the xz_dec_bcj_end and memzero macros
(memeq is left because of negative returns),
-disable XZ_DEC_SINGLE and XZ_DEC_PREALLOC, since we aren't using them.
(160 bytes)
-Merge xz_dec_lzma2_end into xz_dec_end
-Move xz_crc32 to where it's defined.
(That does not seem to be duplicated elsewhere in toybox.)
Convert tabs to spaces.