* cipher/Makefile.am: Remove 'chacha20-sse2-amd64.S',
'chacha20-ssse3-amd64.S', 'chacha20-avx2-amd64.S'; Add
'chacha20-amd64-ssse3.S', 'chacha20-amd64-avx2.S'.
* cipher/chacha20-amd64-avx2.S: New.
* cipher/chacha20-amd64-ssse3.S: New.
* cipher/chacha20-armv7-neon.S: Rewrite.
* cipher/chacha20-avx2-amd64.S: Remove.
* cipher/chacha20-sse2-amd64.S: Remove.
* cipher/chacha20-ssse3-amd64.S: Remove.
* cipher/chacha20.c (CHACHA20_INPUT_LENGTH, USE_SSE2, USE_NEON)
(ASM_EXTRA_STACK, chacha20_blocks_t, _gcry_chacha20_amd64_sse2_blocks)
(_gcry_chacha20_amd64_ssse3_blocks, _gcry_chacha20_amd64_avx2_blocks)
(_gcry_chacha20_armv7_neon_blocks, QROUND, QOUT, chacha20_core)
(chacha20_do_encrypt_stream): Remove.
(_gcry_chacha20_amd64_ssse3_blocks4, _gcry_chacha20_amd64_avx2_blocks8)
(_gcry_chacha20_armv7_neon_blocks4, ROTATE, XOR, PLUS, PLUSONE)
(QUARTERROUND, BUF_XOR_LE32): New.
(CHACHA20_context_s, chacha20_blocks, chacha20_keysetup)
(chacha20_encrypt_stream): Rewrite.
(chacha20_do_setkey): Adjust for new CHACHA20_context_s.
* configure.ac: Remove 'chacha20-sse2-amd64.lo',
'chacha20-ssse3-amd64.lo', 'chacha20-avx2-amd64.lo'; Add
'chacha20-amd64-ssse3.lo', 'chacha20-amd64-avx2.lo'.

STREAM enc |     0.319 ns/B    2988.5 MiB/s      1.28 c/B
STREAM dec |     0.318 ns/B    2995.4 MiB/s      1.27 c/B

STREAM enc |     0.633 ns/B    1507.4 MiB/s      2.53 c/B
STREAM dec |     0.633 ns/B    1506.6 MiB/s      2.53 c/B

STREAM enc |      2.05 ns/B     465.2 MiB/s      8.20 c/B
STREAM dec |      2.04 ns/B     467.5 MiB/s      8.16 c/B

STREAM enc |      5.29 ns/B     180.3 MiB/s      6.09 c/B
STREAM dec |      5.29 ns/B     180.1 MiB/s      6.10 c/B