Fix check of cpuid_asm_src config attribute

The contents of that attribute is C file names, not object file names.
This went undetected because quite a lot of systems have assembler
implementations anyway, so setting OPENSSL_CPUID_OBJ was correct for
them.

Reviewed-by: Andy Polyakov <appro@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/5291)