[LinkerWrapper] Fix resolution of weak symbols during LTO (#68215)

Summary:
Weak symbols are supposed to have the semantics that they can be
overriden by a strong (i.e. global) definition. This wasn't being
respected by the LTO pass because we simply used the first definition
that was available. This patch fixes that logic by doing a first pass
over the symbols to check for strong resolutions that could override a
weak one.

A lot of fake linker logic is ending up in the linker wrapper. If there
were an option to handle this in `lld` it would be a lot cleaner, but
unfortunately supporting NVPTX is a big restriction as their binaries
require the `nvlink` tool.