constify ssl_pad and tls 1.2 lookup tables

Currently 

ssl/s3_enc.c : ssl3_pad_1 and ssl3_pad_2

ssl/t1_lib.c:    tls12_md, tls12_sig, nid_list, suiteb_sigalgs, tls12_sigalgs

All may live in writeable memory, this is a bad idea for both performance and security reasons.