Makefile.in: Add support for running glibc tests

Makefile.in has support for running the regression test suite of
Binutils, GCC and others. Let's add support for running glibc tests.

To run the tests the following command can be used:
  make check-glibc-linux

Tested with linux/rv64.

Signed-off-by: Christoph Müllner <christoph.muellner@vrull.eu>