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.

(wiz)