The u-boot tarball needed for some boards, BEAGLEBONE for
example, explicitly hard-code gcc(1) as the compiler.

Partially revert r264703, which did a post-chroot install
of gcc(1). This was initially removed because gcc(1) fails
to build usr.bin/dtc/ causing the xdev target to fail. So
this time, move the gcc(1) installation after xdev is built.

This change is likely applicable to stable/10 arm build
failures, as well.

MFC after: 3 days
Sponsored by: The FreeBSD Foundation