Export WORLD_FLAGS and KERNEL_FLAGS for buildworld/buildkernel.

Sponsored by:	The FreeBSD Foundation
Pass WORLD_FLAGS to the gcc and xdev builds.

Building gnu/usr.bin/cc/ with '-j' set blows up quite
impressively, so add '-j1' after WORLD_FLAGS, and add
a comment explaining why '-j1' is there.

Sponsored by:	The FreeBSD Foundation