rsa, elgamal: avoid logical not operator in constant-time code
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rsa, elgamal: avoid logical not operator in constant-time code

* cipher/elgamal.c (elg_decrypt): Replace ! operator with calls to
ct_is_not_zero/ct_is_zero/ct_ulong_select.
* cipher/rsa-common.c (_gcry_rsa_pkcs1_decode_for_enc): Replace !
operator with call to ct_is_zero.
* cipher/rsa.c (rsa_decrypt): Replace ! operator with calls to
ct_is_not_zero/ct_is_zero/ct_ulong_select.
* src/const-time.c (_gcry_ct_vzero, _gcry_ct_vone): New.
* src/const-time.h (_gcry_ct_vzero, _gcry_ct_vone): New.
(ct_is_not_zero, ct_is_zero, DEFINE_CT_TYPE_SELECT_FUNC)
(ct_uintptr_select, ct_ulong_select): New.
(sexp_null_cond): Use ct_uintptr_select.

Cherry-pick master commit of:
84f934c09afac18b3f4351646c0fe6f93aede277