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

As the  result of commit 6e6249d0b4
("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>

ChangeLog

@@ -1,3 +1,8 @@
+2019-02-07  Florian Weimer  <fweimer@redhat.com>
+
+	* rt/clock-compat.c (COMPAT_REDIRECT): Turn librt forwarders into
+	compatibility symbols.
+
 2019-02-07  Stefan Liebler  <stli@linux.ibm.com>
 
 	[BZ #24180]

NEWS

@@ -14,7 +14,11 @@ Major new features:
 
 Deprecated and removed features, and other changes affecting compatibility:
 
-  [Add deprecations, removals and changes affecting compatibility here]
+* The functions clock_gettime, clock_getres, clock_settime,
+  clock_getcpuclockid, clock_nanosleep were removed from the librt library
+  for new applications (on architectures which had them).  Instead, the
+  definitions in libc will be used automatically, which have been available
+  since glibc 2.17.
 
 Changes to build and runtime requirements:
 

rt/clock-compat.c

@@ -30,14 +30,16 @@
 #if HAVE_IFUNC
 # undef INIT_ARCH
 # define INIT_ARCH()
-# define COMPAT_REDIRECT(name, proto, arglist) libc_ifunc (name, &__##name)
+# define COMPAT_REDIRECT(name, proto, arglist) libc_ifunc (name, &__##name) \
+    compat_symbol (librt, name, name, GLIBC_2_2);
 #else
 # define COMPAT_REDIRECT(name, proto, arglist)				      \
   int									      \
   name proto								      \
   {									      \
     return __##name arglist;						      \
-  }
+  }									      \
+  compat_symbol (librt, name, name, GLIBC_2_2);
 #endif
 
 COMPAT_REDIRECT (clock_getres,