grpc: fix build on armv6l

pkgs/development/libraries/grpc/default.nix

@@ -1,4 +1,4 @@
-{ stdenv, fetchFromGitHub, cmake, zlib, c-ares, pkgconfig, openssl, protobuf, gflags }:
+{ stdenv, fetchFromGitHub, fetchpatch, cmake, zlib, c-ares, pkgconfig, openssl, protobuf, gflags }:
 
 stdenv.mkDerivation rec {
   version = "1.25.0"; # N.B: if you change this, change pythonPackages.grpcio and pythonPackages.grpcio-tools to a matching version too
@@ -10,6 +10,14 @@ stdenv.mkDerivation rec {
     sha256 = "02nbmbk1xpibjzvbhi8xpazmwry46ki24vks1sh2p0aqwy4hv6yb";
     fetchSubmodules = true;
   };
+  patches = [
+    # Fix build on armv6l (https://github.com/grpc/grpc/pull/21341)
+    (fetchpatch {
+      url = "https://github.com/grpc/grpc/commit/ffb8a278389c8e3403b23a9897b65a7390c34645.patch";
+      sha256 = "1lc12a3gccg9wxqhnwgldlj3zmlm6lxg8dssvvj1x7hf655kw3w3";
+    })
+  ];
+
   nativeBuildInputs = [ cmake pkgconfig ];
   buildInputs = [ zlib c-ares c-ares.cmake-config openssl protobuf gflags ];