[yaml2obj] - Fix .dynamic section entries writing for 32bit targets.

This was introduced by me in r353613.

I tried to fix Big-endian bot and replaced
uintX_t -> ELFT::Xword. But ELFT::Xword is a packed<uint64_t>,
so it is always 8 bytes and that was obviously incorrect.

My intention was to use something like packed<uint> actually, which
size is target dependent.

Patch fixes this bug and adds a test case, since no bots seems reported this.

llvm-svn: 353636