This variable is needed to make toolchain/host/gcc work with e.g.
Ubuntu and other distros.
Without it crt{1,i,n}.o, Scrt.o and libc.so cannot be found.

GCC 4.9.3 seems to leak `CFLAGS` for the compiler itself into
`CFLAGS_FOR_TARGET`, leading to trying to link `libgcc` (and others)
against legacy-glibc.