stdlib: add sys.user module for Linux

sys/user.h provides definitions for registers.  It would previously get
merged into the sys module which could result in it being multiply
included (as in the case of libdispatch).  Add an explicit submodule.
Noticed when building libdispatch on Linux AArch64.

Author: compnerd

stdlib/public/Platform/glibc.modulemap.gyb

@@ -413,6 +413,12 @@ module SwiftGlibc [system] {
         header "${GLIBC_ARCH_INCLUDE_PATH}/sys/un.h"
         export *
       }
+% if CMAKE_SDK in ["LINUX"]:
+      module user {
+        header "${GLIBC_ARCH_INCLUDE_PATH}/sys/user.h"
+        export *
+      }
+% end
       module utsname {
         header "${GLIBC_ARCH_INCLUDE_PATH}/sys/utsname.h"
         export *