[ELF] Un-break undef-from-main-dso.test on MSVC 2012.

The result of sizeof(SymbolTable<ELFT>::SymbolEntry) in DynamicSymbolTable
<ELFT>::write() was different from the same expression in RelocationTable
<ELFT>::write(), although the same template parameters were passed. They were
40 and 32, respectively. As a result, the same vector was treated as a
vector of 40 byte values in some places and a vector of 32 values in other
places. That caused an weird issue, resulting in collapse of the rela.dyn
section.

I suspect that this is a padding size calculation bug in MSVC 2012, but I
may be wrong. Reordering the fields to eliminate padding seems to fix the
issue.