gcc-target: Don't install target gcc libdir files

Installing /usr/lib/gcc/* means we'd have two copies, one from gcc-cross
and one from here. These can confuse gcc cross where includes use #include_next
and builds track file dependencies (e.g. perl and its makedepends code).
For determinism we don't install this to the sysroot, ever and rely on the
copy from gcc-cross.

[YOCTO #7287]

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>