Ignore relocation tables for non-memory-resident sections.

As a followup to r328101, ignore relocation tables for ELF object
sections that are not memory resident. For modules loaded by the
loader, ignore relocation tables whose associated section was not
loaded by the loader (sh_addr is zero). For modules loaded at runtime
via kldload(2), ignore relocation tables whose associated section is
not marked with SHF_ALLOC.

Reported by: Mori Hiroki <yamori813@yahoo.co.jp>, adrian
Tested on: mips, mips64
MFC after: 1 month
Sponsored by: DARPA / AFRL