Some "distclean" fixes in gdb

Some "distclean" fixes in gdb

PR build/12440 points out that "make distclean" is broken in gdb.
Most of the breakage comes from other projects in the tree, but we can
fix some of the issues, which is what this patch does.

Note that the yacc output files, like c-exp.c, are left alone.  In a
source distribution, these are included in the tarball, and if the
user builds in-tree, we would not want to remove them.

While that seems a bit obscure, it seems to me that "distclean" is
only really useful for in-tree builds anyway -- out-of-tree I simply
delete the entire build directory and start over.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=12440