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

--- swift/stdlib/public/Platform/CMakeLists.txt	2019-04-09 20:14:44.493801403 +0200
+++ swift/stdlib/public/Platform/CMakeLists.txt	2019-04-09 20:14:44.577800593 +0200
@@ -77,7 +77,7 @@
     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_${arch}_PATH}" STREQUAL "/" AND NOT "${sdk}" STREQUAL "ANDROID")
       set(GLIBC_INCLUDE_PATH "${SWIFT_SDK_${sdk}_ARCH_${arch}_PATH}${GLIBC_INCLUDE_PATH}")