libdwarf: Hide SHT_NOBITS sections.

gnu_debuglink external debug files will contain an .eh_frame section of
type SHT_NOBITS. libdwarf does not handle such sections (or rather, it
expects all debug sections to not have type SHT_NOBITS). Avoid loading
SHT_NOBITS sections, to be consistent with SGI libdwarf's handling of
this case.

PR: 239516
Diagnosed by: Paco Pascal <me@pacopascal.com>
Reviewed by: emaste (previous version)
Event: July 2020 Bugathon
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D25626