Windows, VMS: build fixes

The naming of generated assembler wasn't done quite right.  There are
assembler files that are generated from a perl script, and there are
those who are not.  Only the former must be renamed to the platform
specific asm extension.

Furthermore, we need to make sure that 'OSSL_provider_init' isn't case
sensitive on VMS, to allow for the least surprise for provider
builders.

Reviewed-by: Tim Hudson <tjh@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/8529)