[gdb/symtab] Factor out compunit_epilogue_unwind_valid

[gdb/symtab] Factor out compunit_epilogue_unwind_valid

Factor out compunit_epilogue_unwind_valid from both
amd64_stack_frame_destroyed_p and i386_stack_frame_destroyed_p.  No functional
changes.

Also add a comment in the new function about the assumption that in absence of
producer information, epilogue unwind info is invalid.

Approved-By: Tom Tromey <tom@tromey.com>