crypto/internal/nistec: re-enable s390x asm for P-256

crypto/internal/nistec: re-enable s390x asm for P-256

- formatting and optimized init functionality for precomputed table
- updated formatting for comments
- further optimized init functionality
Fixes #52709

Change-Id: Ie96a8ee52f09821d5ac53115185cbc1ad8f954d2
Reviewed-on: https://go-review.googlesource.com/c/go/+/404058
Auto-Submit: Lynn Boger <laboger@linux.vnet.ibm.com>
Reviewed-by: Lynn Boger <laboger@linux.vnet.ibm.com>
Run-TryBot: Lynn Boger <laboger@linux.vnet.ibm.com>
Reviewed-by: Bill O'Farrell <billotosyr@gmail.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Pooja Shyamsundar <poojashyam@ibm.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Reviewed-by: Filippo Valsorda <filippo@golang.org>