[yaml2obj] Rename SHOffset (e_shoff) field to SHOff. NFC

`struct Elf*_Shdr` has a field `sh_offset`, named `ShOffset` in
llvm::ELFYAML::Section. Rename SHOffset (e_shoff) to SHOff to prevent confusion.

Reviewed By: grimar

Differential Revision: https://reviews.llvm.org/D67254

llvm-svn: 371185