[lto] Fix symlookup in archives vs shared

when a shared library defines 'foo@@FOO' (default version),
a static archive defines 'foo', the shared lib comes in front
of the archive and under effect of --as-needed, and the requesting
object file uses LTO, then the link editor was wrongly including
the definition from the static archive.  It must use the one
from the shared lib, like in the non-LTO or the --no-as-needed case.
See the added testcase that would wrongly print "FAIL" before
this patch.

The problem stems from several connected problems:
(1) only the decorated symbol was entered into first_hash (the hash
    table designed to handle definition order in the pre-LTO-plugin
    phase of the symbol table walks)
(2) in the archive symbol walk only the undecorated name would be
    looked up in first_hash (and hence not found due to (1))
(3) in the archive symbol walk first_hash would only be consulted
    when the linker hash table had a defined symbol.  In pre-LTO
    phase shared lib symbols aren't entered into the linker symbol
    table.

So: add also the undecorated name into first_hash when it stems from
a default version and consult first_hash in the archive walker also
for currently undefined symbols.  If it has an entry which doesn't
point to the archive, then it comes from an earlier library (shared or
static), and so _this_ archive won't provide the definition.

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