sha1: Use bswap* to optimize byte order

bionic libc already makes use of ARMv6+ rev/rev16 instruction for
endian conversion, and this patch rewrites some parts of SHA1
implementations with swap32 and swap64 routines, which is known to
bring performance improvements.

The reference sha1bench on Nexus S:

[before]
Rounds: 100000, size: 6250K, time: 1.183s, speed: 5.16  MB/s

Change-Id: Id04c0fa1467b3006b5a8736cbdd95855ed7c13e4
[after]
Rounds: 100000, size: 6250K, time: 1.025s, speed: 5.957 MB/sB