libglib2: backport native linux GMutex implementation

This fixes libglib2 build on architectures that don't support NPTL.

Fixes:
  http://autobuild.buildroot.org/results/a1a/a1a1f97a2c54ba4f7f7e44cc094a55bd23ca0aa0/



Backported from: 49b59e5ac4428a6a99a85d699c3662f96efc4e9d
Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>