Add --enable-fat support for arm neon chacha20

On BCM2837B0 (Cortex-A53) @1.4GHz (Raspberry Pi 3B+),
Before:
`gnutls-cli --benchmark-ciphers`
       CHACHA20-POLY1305 (16384) 51.54 MB/sec
`gnutls-cli --benchmark-tls-ciphers`:
       ECDHE_RSA_CHACHA20_POLY1305 (payload 1400)  21.31 MB/sec
       ECDHE_RSA_CHACHA20_POLY1305 (payload 15360)  24.60 MB/sec
`nettle-benchmark`
		 chacha     encrypt   71.90
		 chacha     decrypt   71.89
	chacha_poly1305     encrypt   48.17
	chacha_poly1305     decrypt   48.17
	chacha_poly1305      update  146.03

After:
`gnutls-cli --benchmark-ciphers`
       CHACHA20-POLY1305 (16384) 68.44 MB/sec
`gnutls-cli --benchmark-tls-ciphers`:
       ECDHE_RSA_CHACHA20_POLY1305 (payload 1400) 27.25 MB/sec
       ECDHE_RSA_CHACHA20_POLY1305 (payload 15360) 32.41 MB/sec
`nettle-benchmark`
		 chacha     encrypt  106.00
		 chacha     decrypt  105.94
	chacha_poly1305     encrypt   65.94
	chacha_poly1305     decrypt   65.96
	chacha_poly1305      update  175.24