[ELF] Move fixSectionAlignments() before first call to assignAddresses()

The fixSectionAlignments() function may alter the alignment of some
OutputSections, this is likely to alter the addresses calculated earlier
in assignAddresses(). By moving the call to fixSectionAlignments() we
make sure that assignAddresses() is consistent with the early calculation
used for RangeThunks and the final call just before writing the image.

Differential Revision: https://reviews.llvm.org/D36739

llvm-svn: 312636