-use-linker-plugin passed to ld

[Ian Lance Taylor]

I noticed that the -use-linker-plugin option seems to be passed to the
linker.  This is because LINK_COMMAND_SPEC includes %{u*}.  And that
is because -uSYMBOL is a documented linker option.

The effect is that the linker creates an undefined reference to the
symbol "se-linker-plugin".  Any user unfortunate enough to link
against a library which contains a definition of that symbol may see
an unexpected result.  Admittedly unlikely, but still.

I think we need to fix this option before the release to not overlap
with any existing linker options.

Ian