self_reloc.c: Pass -Wno-error=maybe-uninitialized to gcc versions greater than 4.2.1

self_reloc.c doesn't initialize rel in all cases in the C code, however, the value
might be initialized properly on the stack in the assembly code.

For right now (because this doesn't seem to be breaking anything and my initializing
the stack value could break something since it's called from assembly code) disable
the warning for self_reloc.c. More investigation should be done to determine the
appropriate response to this warning (either intialize the value or find a smarter
way to deal with the warning).

A long MFC timeout is being set for this change to allow a better solution for the
issue to be developed in that time period.

MFC after: 2 months
Reported by: Jenkins (FreeBSD-head-amd64-gcc job)
Tested with: amd64-gcc-6.3.0 (devel/amd64-xtoolchain-gcc)
Sponsored by: Dell EMC Isilon