[lld][ELF] Add parser for linker script ENTRY command

This is the parser for the ENTRY command. Note that because the parsing result
is currentlyd discarded, lld can parse but just ignore the command.

Reviewers: Bigcheese

CC: llvm-commits

Differential Revision: http://llvm-reviews.chandlerc.com/D833

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@183141 91177308-0d34-0410-b5e6-96231b3b80d8