i386: expand exception 3 DO_TRAP macro

The int3 exception was already takes as an interrupt and
do_int3 does not fit in the new DO_ERROR macro. This patch
just expands the DO_TRAP macro and rearranges the code a
bit.

No functional changes intended.

Signed-off-by: Alexander van Heukelum <heukelum@fastmail.fm>
Signed-off-by: Ingo Molnar <mingo@elte.hu>