Fix Configure test for -mips in CFLAGS

Fix Configure test for -mips in CFLAGS

We want to add -mips2 or -mips3 only if the user hasn't already
specified a mips version in CFLAGS. The existing test was a
double-negative.

CLA: trivial

Fixes: https://github.com/openssl/openssl/issues/20214
Reviewed-by: Matt Caswell <matt@openssl.org>
Reviewed-by: Shane Lontis <shane.lontis@oracle.com>
Reviewed-by: Paul Dale <pauli@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/20536)