Convert SHA-1 SSSE3 implementation from mixed asm&C to pure asm

* cipher/Makefile.am: Change 'sha1-ssse3-amd64.c' to
'sha1-ssse3-amd64.S'.
* cipher/sha1-ssse3-amd64.c: Remove.
* cipher/sha1-ssse3-amd64.S: New.
--

Mixed C&asm implementation appears to trigger GCC bugs easily. Therefore
convert SSSE3 implementation to pure assembly for safety.

Benchmark also show smallish speed improvement.

cpu             C&asm     asm
Intel i5-4570   5.22 c/B  5.09 c/B
Intel i5-2450M  7.24 c/B  7.00 c/B

Signed-off-by: Jussi Kivilinna <jussi.kivilinna@iki.fi>
SHA-1/SSSE3: Do not check for Intel syntax assembly support

* cipher/sha1-ssse3-amd64.S: Remove check for
HAVE_INTEL_SYNTAX_PLATFORM_AS.
* cipher/sha1.c [USE_SSSE3]: Ditto.
--

SHA-1 SSSE3 implementation uses AT&T syntax so check for
HAVE_INTEL_SYNTAX_PLATFORM_AS is unnecessary.

Signed-off-by: Jussi Kivilinna <jussi.kivilinna@iki.fi>