Add libc ABI extension kludge for baseline-violating libdl symbols

Some targets have a GLIBC_2.0 baseline for libdl, while using
GLIBC_2.2 for libc.  This means that the generated libc.map file
does not have any version nodes for GLIBC_2.0 or GLIBC_2.1.  However,
moving symbols from libdl into libc needs such version nodes.
(Future symbol moves from librt will need this as well.)

This kludge is only necessary for symbols predating GLIBC_2.2 because
the affected targets use GLIBC_2.2 as the baseline for libc.  Given
the small number and fixed set of affected architectures, no generic
mechanism is implemented, and instead the map file fragment is
hard-coded in scripts/versions.mk.

The compat_symbol macro already emits the appropriate version strings,
so no adjustments are needed there.

Reviewed-by: Adhemerval Zanella  <adhemerval.zanella@linaro.org>

sysdeps/unix/sysv/linux/sparc/sparc64/Versions

@@ -1,3 +1,4 @@
+!libc_pre_versions
 libc {
   GLIBC_2.0 {
     # Exception handling support functions from libgcc