AES-XTS on AArch64: Set w19 earlier before cipher-stealing of 1 block + tail.

[nebeid]

Description of changes:

w19 was set to its fixed value 0x87 after the code jumps to the case of cipher stealing of 1 block and a tail.

• This bug was introduced in v1.62 in "Move udiv and sencond tweak calculations to when needed" by @nebeid in #2726 where the setting of w19 was moved past the branch to .Lxts_enc_tail1x.
• Also the 3 test vectors that tested 1 block + tail were not lucky enough to exercise this issue which happens when the encrypted tweak, when shifted left by 1, results in a bit of 1, which, in turn, results in XORing the value 0x87 with the tweak.

Testing:

The failing tests are now passing.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license and the ISC license.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 78.52%. Comparing base (03d44c8) to head (5e4a01f).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2785   +/-   ##
=======================================
  Coverage   78.52%   78.52%           
=======================================
  Files         678      678           
  Lines      115344   115344           
  Branches    16327    16330    +3     
=======================================
  Hits        90572    90572           
+ Misses      23978    23977    -1     
- Partials      794      795    +1     

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