NO_IMPLICIT_EXTERN_C is not set - leads to C++ compilation errors

Steps to reproduce:
$ mkdir foo
$ echo "template<typename C> struct Blubber {};" > foo/foo.h
$ echo "#include \"foo.h\"" > foo.cpp
$ arm-none-eabi-g++ -isystem /home/andreas/foo/ foo.cpp -o FOO
In file included from foo.cpp:1:0:
/home/andreas/foo/foo.h:1:1: error: template with C linkage
 template<typename C> struct Blubber {};
 ^~~~~~~~

$ lsb_release -rd
Description: Ubuntu 18.04.1 LTS
Release: 18.04

$ apt-cache policy gcc-arm-none-eabi
gcc-arm-none-eabi:
  Installed: 15:6.3.1+svn253039-1build1
  Candidate: 15:6.3.1+svn253039-1build1
  Version table:
 *** 15:6.3.1+svn253039-1build1 500
        500 http://de.archive.ubuntu.com/ubuntu bionic/universe amd64 Packages
        100 /var/lib/dpkg/status

The compiler package is usually used in conjunction with newlib and libstdc++-arm-none-eabi-newlib so the C library being used is c++ aware.