Implement BLAKE2s with the same macro as BLAKE2b

Implement BLAKE2s with the same macro as BLAKE2b

This avoids code duplication and provides variable-size support
for BLAKE2s like 786b9a8

Test data obtained with libb2 with the following programs:

	==> b2.c <==
	#include <blake2.h>
	#include <unistd.h>

	int main() {
		char buf[16] = {};
		blake2s(buf, 0, 0, 16, 0, 0);
		write(1, buf, 16);
	}

	==> b3.c <==
	#include <blake2.h>
	#include <unistd.h>

	int main() {
		char buf[10] = {};
		blake2s(buf, "\x61", 0, 10, 1, 0);
		write(1, buf, 10);
	}

Reviewed-by: Paul Dale <pauli@openssl.org>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/22444)