MCSymbol: Remove AMDGPU-specific Kind::TargetCommon
The SymContentsTargetCommon kind introduced by
https://reviews.llvm.org/D61493 lackes significant and should be treated
as a regular common symbol with a different section index.
Update ELFObjectWriter to respect the specified section index.
The new representation also works with Hexagon's SHN_HEXAGON_SCOMMON.
