Makefile: BLK_SHA1 does not require fast htonl() and unaligned loads

Makefile: BLK_SHA1 does not require fast htonl() and unaligned loads

block-sha1/ is fast on most known platforms.  Clarify the Makefile to
be less misleading about that.

Early versions of block-sha1/ explicitly relied on fast htonl() and
fast 32-bit loads with arbitrary alignment.  Now it uses those on some
arches but the default behavior is byte-at-a-time access for the sake
of arches like ARM, Alpha, and their kin and it is still pretty fast
on these arches (fast enough to supersede the mozilla SHA1
implementation and the hand-written ARM assembler implementation that
were bundled before).

Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>