[LLD] [COFF] Support ENTRY and SUBSYSTEM in .drectve sections

Adds support for "/ENTRY" and "/SUBSYSTEM" linker options in .drectve
sections. Some Mozilla binaries were using these directives and MSVC
link.exe appears to allow them. No attempt is made to reconcile these
with the options on the command line.

Patch by David Major!

Differential Revision: https://reviews.llvm.org/D39972

llvm-svn: 319356