toolchain: only fetch gcc/gmp/mpfr/mpc when using a buildroot toolchain

Closes #3103

gcc/gmp/mpfr/mpc are only needed when using an internal buildroot toolchain,
so only add them to HOST_SOURCE then.

Otherwise we get some nasty fetch error when doing "make source"

[Peter: add gmpc/mpfr/mpc and reword commit text]
Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>