The Nix glibc headers do not use include/x86_64-linux-gnu subdirectories.

--- swift/stdlib/public/Platform/CMakeLists.txt	2018-09-30 17:51:51.581766303 +0200
+++ swift/stdlib/public/Platform/CMakeLists.txt	2018-09-30 18:40:04.118956708 +0200
@@ -65,7 +65,7 @@
     endif()
 
     set(GLIBC_INCLUDE_PATH "${GLIBC_SYSROOT_RELATIVE_INCLUDE_PATH}")
-    set(GLIBC_ARCH_INCLUDE_PATH "${GLIBC_SYSROOT_RELATIVE_ARCH_INCLUDE_PATH}")
+    set(GLIBC_ARCH_INCLUDE_PATH "${GLIBC_SYSROOT_RELATIVE_INCLUDE_PATH}")
 
     if(NOT "${SWIFT_SDK_${sdk}_ARCH_${arch}_PATH}" STREQUAL "/")
       set(GLIBC_INCLUDE_PATH "${SWIFT_SDK_${sdk}_ARCH_${arch}_PATH}${GLIBC_INCLUDE_PATH}")