new .fixup group size algorithm

The fixup_group_size() function assumes that all .fixup rela groups end
with a jmpq instruction.  That assumption turns out to be false when you
take into account the ____kvm_handle_fault_on_reboot() macro which is
used by kvm.

This is a new, more reliable method.  It turns out that each .fixup
group is referenced by the __ex_table section.  The new algorithm goes
through the __ex_table relas to figure out the size of each .fixup
group.

Also the .fixup section is now processed before __ex_table, because it
needs to access the original __ex_table relas before the unused ones
have been stripped.

Fixes the following error:

  ERROR: vmx.o: fixup_group_size: 1554: can't find jump instruction in .fixup section
test: add slow test for rebuilding entire tree

Only gets run with the slow integration tests, not "make quick".