dev-cpp/abseil-cpp: backport fix for arm64 CPUs without crypto extensions

* Backport fix for arm64 CPUs without crypto extensions
  (needed for a keywording bug).

* Add information in patch / background on the problem
  (including mentioning the need for an ebuild change,
  in case e.g. another distro tries to pick it up,
  and to remind us to try upstream it.)

* Add explicit flag-o-matic inherits accordingly.

See: 619cfe3dae
Bug: https://bugs.gentoo.org/778926
Bug: https://bugs.gentoo.org/766411
Signed-off-by: Sam James <sam@gentoo.org>

dev-cpp/abseil-cpp/files/abseil-cpp-20200923-arm_no_crypto.patch

@@ -1,3 +1,10 @@
+Don't assume all ARM CPUs have crypto extensions.
+
+Requires an accompanying change in the ebuild to disable crypto
+extensions (by passing -DABSL_ARCH_ARM_NO_CRYPTO). This could
+be done by the build system instead in future.
+
+https://bugs.gentoo.org/778926
 --- /absl/random/internal/randen_hwaes.cc
 +++ /absl/random/internal/randen_hwaes.cc
 @@ -50,8 +50,11 @@