bn_exp.c: move check for AD*X to rsaz-avx2.pl.

This ensures high performance is situations when assembler supports
AVX2, but not AD*X.
Remove ancient obsolete files under pkcs7.
Make sure BN_sqr can never return a negative value.

PR#3410
Set up Solano CI build

Thanks to William Josephson at Solano Labs for generously volunteering
Solano's services.