dfc/extern: Reorder structure fields.

The previous layout of the structure wastes 3 bytes between `ghigh` and
`gsymbol`. Consequently, the whole structure can no longer fit into the
an L1 cache line.
The real effect is probably minor in our case, but still...

Signed-off-by: Sylvain Laperche <sylvain.laperche@gmail.com>