src.opts.mk: disable BINUTILS_BOOTSTRAP on mips64*

After GCC was disabled by default in r356367, mips and sparc64 started
relying external GCC. However, the in-tree Binutils ld 2.17.50 is not
compatible with GCC for some mips64 targets, so turn off
BINUTILS_BOOTSTRAP and rely on external binutils (linker) as well.