build tools form part of "corresponding source code"

I believe (but IANAL) that in this scenario any required nonstandard build tool, to wit the GCC plugin, are considered to be part and parcel of the source code that corresponds to the supplied binaries. This is just the same as requiring preprocessors and build scripts to be distributed alongside the source.

The normal compiler and "system libraries" are exempt in normal circumstances thanks to this language (GPL v2, section 3): "as a special exception, the source code distributed need not include anything that is normally distributed (in either source or binary form) with the major components (compiler, kernel, and so on) of the operating system on which the executable runs".

As far as I can tell, there is nothing magic about a non-standard compiler (or compiler plugin) that exempts it from this requirement.

Of course that doesn't mean that the compiler itself *is* part of the source -- just that it's impossible to fulfil the requirement to distribute source without it, if the compiler is unusual.