Switch x86_64 asm to use "i" instead of "n" for immediate values.

Summary:
The constraint "n" is a more obscure setting which is needed for
special cases that don't apply to our use.

Use of "n" exposed some bugs in pre-release versions of clang.
This fixes #594

This is a backport of secp256k1's PR599

Test Plan:

make check

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D4989