Log Message:
libgcrypt: update to 1.8.6.

Noteworthy changes in version 1.8.6 (2020-07-06)  [C22/A2/R6]
------------------------------------------------

 * Bug fixes:

   - Fix build problems on OpenIndiana et al. [#4818]

   - Fix GCM bug on arm64 which troubles for example OMEMO.  [#4986]

   - Fix wrong code execution in Poly1305 ARM/NEON implementation.
     [#4833]

   - Detect a div-by-zero in a debug helper tool.  [#4868]

   - Use a constant time mpi_inv in some cases and change the order
     mpi_invm is called.  [#4869]

   - Fix mpi_copy to correctly handle flags of opaque MPIs.

   - Fix mpi_cmp to consider +0 and -0 the same.

 * Other features:

   - Add OIDs from RFC-8410 as aliases for Ed25519 and Curve25519.