ppc: New assembly for chacha_core4, doing four blocks in parallel.

* chacha-crypt.c: (_nettle_chacha_crypt_4core)
(_nettle_chacha_crypt32_4core): New functions.
* chacha-internal.h: Add prototypes for _nettle_chacha_4core and
related functions.
* configure.ac (asm_nettle_optional_list): Add chacha-4core.asm.
* powerpc64/fat/chacha-4core.asm: New file.
* powerpc64/p7/chacha-4core.asm: New file.
* fat-ppc.c (fat_init): When altivec is available, use
_nettle_chacha_crypt_4core and _nettle_chacha_crypt32_4core
instead of _2core variants.