Simplify writeThunk. NFC.
Simplify writeThunk. NFC.

Previously, `Buf + 4` was written twice.

git-svn-id: f6089bf0e6284f307027cef4f64114ee9ebb0424@273337 91177308-0d34-0410-b5e6-96231b3b80d8

llvm-mirror/lld,llvm-mirror/lld