webkit-gtk: Set USE_PKGSRC_GCC_(RUNTIME) when building on NetBSD 9

We likely need this anyway since WebKit requires GCC 8 at minimum,
but forcing pkgsrc libgcc to be used (and thus bypassing the one
built with the custom build system in NetBSD that doesn't include
__fixdfti on aarch64) may be a useful workaround for PR toolchain/57022

(nia)