MAINTAINERS: Add x86 unwinding entry

Create a new section for x86 unwinder maintenance.

Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com>
Link: https://lore.kernel.org/r/db2b764b735a9481df9f7717a3a1f75ba496fcc1.1650387176.git.jpoimboe@redhat.com


Signed-off-by: Peter Zijlstra <peterz@infradead.org>