Remove some inline assembler and non-standard constructs.

The goal is to minimize maintenance burden by eliminating somewhat
obscure platform-specific tweaks that are not viewed as critical for
contemporary applications. This affects Camellia and digest
implementations that rely on md32_common.h, MD4, MD5, SHA1, SHA256.
SHA256 is the only one that can be viewed as critical, but given
the assembly coverage, the omission is considered appropriate.

Reviewed-by: Rich Salz <rsalz@openssl.org>
Reviewed-by: Richard Levitte <levitte@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/6508)