toolchain/gcc: add openmp support option

Add OpenMP support in gcc as an option instead of dynamically enabling
it when threads are present.
Also fix it up since we never installed libgomp to the target or staging
thus it was never really functional.

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>