Fix gdb.opt/inline-cmds.exp regressions

Fix gdb.opt/inline-cmds.exp regressions

When sorting pending blocks in end_symtab_get_static_block, blocks
with the same starting address must remain in the original order
to preserve inline function caller/callee relationships.

The original code seems to have implicitly relied on the fact that the
glibc qsort implemention actually (in the common case) provides a stable
sort, although this is not guaranteed by the standard.  But the GNU
libstdc++ std::sort implementation is *not* stable.

gdb/ChangeLog:
2017-10-24  Ulrich Weigand  <uweigand@de.ibm.com>

	* buildsym.c (end_symtab_get_static_block): Use std::stable_sort.