boost: fix build for s390

Boost seems to call both S390 and S390x "s390x".  At least that's how
Conan does it:

conan-io/conan-center-index#8282

pkgs/development/libraries/boost/generic.nix

@@ -70,7 +70,7 @@ let
     "address-model=${toString stdenv.hostPlatform.parsed.cpu.bits}"
     "architecture=${if stdenv.hostPlatform.isMips64
                     then if lib.versionOlder version "1.78" then "mips1" else "mips"
-                    else if stdenv.hostPlatform.parsed.cpu.name == "s390x" then "s390x"
+                    else if stdenv.hostPlatform.isS390 then "s390x"
                     else toString stdenv.hostPlatform.parsed.cpu.family}"
     # env in host triplet for Mach-O is "macho", but boost binary format for Mach-O is "mach-o"
     "binary-format=${if stdenv.hostPlatform.isMacho then "mach-o"