rt: Turn forwards from librt to libc into compat symbols [BZ #24194]

As the  result of commit 6e6249d0
("BZ#14743: Move clock_* symbols from librt to libc."), in glibc 2.17,
clock_gettime, clock_getres, clock_settime, clock_getcpuclockid,
clock_nanosleep were added to libc, and the file rt/clock-compat.c
was added with forwarders to the actual implementations in libc.
These forwarders were wrapped in

#if SHLIB_COMPAT (librt, GLIBC_2_2, GLIBC_2_17)

so that they are not present for newer architectures (such as
powerpc64le) with a 2.17 or later ABI baseline.  But the forwarders
were not marked as compatibility symbols.  As a result, on older
architectures, historic configure checks such as

AC_CHECK_LIB(rt, clock_gettime)

still cause linking against librt, even though this is completely
unnecessary.  It also creates a needless porting hazard because
architectures behave differently when it comes to symbol availability.

Reviewed-by: Carlos O'Donell <carlos@redhat.com>