Rename files in providers/implementations/signatures

Rename files in providers/implementations/signatures

It was discovered that eddsa.c exist in two places, here and in
crypto/ec/curve448/, which would result in a file name clash if they
ever end up in the same library.

To mitigate, we rename the copy in providers/implementations/signatures
to have '_sig' in the file name, and do the same with all other source
files in this directory, for consistency.

Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/15171)