[testsuite] don't use lto plugin if it doesn't work

[Mike Stump]

On Jun 27, 2012, at 2:07 AM, Alexandre Oliva wrote:
> Why?  We don't demand a working plugin.  Indeed, we disable the use of
> the plugin if we find a linker that doesn't support it.  We just don't
> account for the possibility of finding a linker that supports plugins,
> but that doesn't support the one we'll build later.

If this is the preferred solution, then having configure check the 64-bitness of ld and turning off the plugin altogether on mismatches sounds like a reasonable course of action to me.