src.opts.mk: disable BINUTILS_BOOTSTRAP on all mips* and sparc64

Extend r356379 to include 32-bit mips and sparc64. Using a decade-old
binutils linker with a contemporary compiler (either Clang or GCC) is
a combination unlikely to be used by anyone else, and it's not going
to be a good use of our time investigating and addressing any issues
that arise. Expect that all architectures newly migrated to external
GCC will also use external binutils.