Exclude sha1.c when building libdex for device

In bionic, there is already an optimized SHA-1 hash implementation,
and the file 'sha1.c' in libdex is almost identical.  It is only
necessary to build sha1.c for host version of libdex.

In addition, the SHA-1 hash implementation in bionic is slightly faster
than the one included in libdex.

Here is the reference SHA-1 benchmark on Nexus S:

[libdex]
Rounds: 100000, size: 6250K, time: 1.28s,  speed: 4.768 MB/s

[bionic]
Rounds: 100000, size: 6250K, time: 1.184s, speed: 5.156 MB/s

(manually cherry-picked commit 55ac5b0a908b85c050f34dc603f62dd4b1fc4e9f)

Change-Id: If97956c4f2a6dc14460edd40853cf5ea6cb54999