Restore clock_* librt exports for MicroBlaze (bug 21061).

MicroBlaze had clock_* functions exported from librt in glibc 2.18 and
2.19, as confirmed in
<https://sourceware.org/ml/libc-alpha/2017-01/msg00369.html>, and they
then disappeared in 2.20, presumably as a result of the fix
<https://sourceware.org/ml/libc-alpha/2014-02/msg00598.html> for a
Versions.def bug that had resulted in their unintended inclusion in
2.18 (followed by removal of the Versions.def mechanism that allowed
such bugs).

As they were released in that library, they should be considered part
of the GLIBC_2.18 ABI and so restored for the sake of any binaries
that expect them in that library.  This patch restores them by adding
a MicroBlaze version of clock-compat.c that overrides SHLIB_COMPAT.

Tested (compilation only) with build-many-glibcs.py (where this fixes
the librt ABI test failure; elf/check-execstack still fails and still
needs architecture maintainer attention to fix it or XFAIL it with an
appropriate explanatory comment).

	[BZ #21061]
	* sysdeps/unix/sysv/linux/microblaze/clock-compat.c: New file.