create-diff-object: percpu support

The -fdata-sections gcc flag doesn't work with objects in the
.data..percpu section.  Any function which uses a percpu variable
references this section, causing the section to get incorrectly included
in the patch module.

Manually convert these section references to object symbol references so
that the needed symbol can be found in vmlinux.

Also, the core module symbol verification code will fail when looking up
a percpu variable, because sprint_symbol doesn't think a percpu address
is a valid kernel address.  So rewrite the symbol verification code to
use kallsyms_on_each_symbol() instead.  It's not ideal performance-wise:
it seems to cost about 1ms per symbol lookup.  I think that's acceptable
for now.  In the future we may want to try to get a better upstream
kallsyms interface.
create-diff-object: fix symbol section indexes

For a local non-included function or object which is needed by an
included function, its symbol table entry will still refer to a local
section index.  Instead it should be changed to SHN_UNDEF.
create-diff-object: improve error message
create-diff-object: generic special section support

Abstract out the common functionality for dealing with special sections
into a new kpatch_process_special_sections() function.

The base sections are partitioned into "groups".  Only those groups
whose relas reference a changed function are kept.  The only difference
in the logic for handling each special section is determining the size
of a given group.  Each section has its own group_size() callback for
this.  It's a callback instead of an integer because one of the
soon-to-be-supported special sections requires that its group sizes be
dynamically determined.
create-diff-object: support fixup and related sections

Add support for the following sections:

- .fixup
- __ex_table
- .altinstructions
- .altinstr_replacement

Fixes #187.
create-diff-object: support tracepoints and related sections

Add support for the following special sections:

- __jump_table
- __tracepoints
- __tracepoints_ptrs
- __tracepoints_strings

Fixes #157.