objtool: Fix infinite loop in find_jump_table()

Kristen found a hang in objtool when building with -ffunction-sections.

It was caused by evergreen_pcie_gen2_enable.cold() being laid out
immediately before evergreen_pcie_gen2_enable().  Since their "pfunc" is
always the same, find_jump_table() got into an infinite loop because it
didn't recognize the boundary between the two functions.

Fix that with a new prev_insn_same_sym() helper, which doesn't cross
subfunction boundaries.

Reported-by: Kristen Carlson Accardi <kristen@linux.intel.com>
Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/378b51c9d9c894dc3294bc460b4b0869e950b7c5.1588110291.git.jpoimboe@redhat.com