objtool: Add support for C jump tables

Objtool doesn't know how to read C jump tables, so it has to whitelist
functions which use them, causing missing ORC unwinder data for such
functions, e.g. ___bpf_prog_run().

C jump tables are very similar to GCC switch jump tables, which objtool
already knows how to read.  So adding support for C jump tables is easy.
It just needs to be able to find the tables and distinguish them from
other data.

To allow the jump tables to be found, create an __annotate_jump_table
macro which can be used to annotate them.

The annotation is done by placing the jump table in an
.rodata..c_jump_table section.  The '.rodata' prefix ensures that the data
will be placed in the rodata section by the vmlinux linker script.  The
double periods are part of an existing convention which distinguishes
kernel sections from GCC sections.

Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Song Liu <songliubraving@fb.com>
Cc: Kairui Song <kasong@redhat.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Alexei Starovoitov <ast@kernel.org>
Cc: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lkml.kernel.org/r/0ba2ca30442b16b97165992381ce643dc27b3d1a.1561685471.git.jpoimboe@redhat.com


Signed-off-by: Ingo Molnar <mingo@kernel.org>