libiberty: fix --enable-install-libiberty flag [PR 56780]

Commit 199570 fixed the --disable-install-libiberty behavior, but it also
added a bug where the enable path never works because the initial clear
of target_header_dir wasn't deleted.  So we end up initializing properly
at the top only to reset it at the end all the time.

git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/trunk@206367 138bc75d-0d04-0410-961f-82ee72b054a4
(cherry picked from commit 369be6981b26787b2685e3b8c6da779dae8ce35f)
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