elf: Implement tail merging of strings in ldconfig

elf: Implement tail merging of strings in ldconfig

This simplifies the string table construction in elf/cache.c
because there is no more need to keep track of offsets explicitly;
the string table implementation does this internally.

This change slightly reduces the size of the cache on disk.  The
file format does not change as a result.  The strings are
null-terminated, without explicit length, so tail merging is
transparent to readers.

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