USE_TOOLS+=sed:run in lang/gcc*

[Taylor R Campbell <campbell+netbsd-tech-pkg%mumble.net@localhost>]

lang/gcc* has had USE_TOOLS+=sed:run since dmcmahill imported
lang/gcc44 in 2009.

Does gcc actually rely on running sed at runtime?

I think it should be USE_TOOLS+=sed instead.

I ran into this because having both :run and :build on the same tool
in USE_TOOLS currently causes trouble for cross-compilation -- this is
a bug that should be fixed, of course, but I suspect gcc actually
needs sed only at build-time, not at run-time.