[ELF] - Assign proper values for DefinedSynthetic symbols attached to non-allocatable sections.

DefinedSynthetic symbols are attached to sections,
for the case when such symbol was attached to non-allocated section,
we calculated its value incorrectly.

We subtracted Body->Section->Addr, but non-allocatable sections
should have zero VA in output and therefore result value was wrong.

And at the same time we have Body->Section->Addr != 0 for them 
internally because  use it for calculation of section size.

Patch fixes calculation of such symbols values.

Differential revision: https://reviews.llvm.org/D29653

llvm-svn: 294322