[COFF] Set MajorLinkerVersion to 14 instead of 0.

This works around a strange interaction with Authenticode signatures,
in which a signed PE executable with {Major,Minor}LinkerVersion = 0.0
fails to validate on Windows 7 (but is OK on Windows 10). Setting the
linker version to 14.0 (which is what VS2015 outputs) makes it work
again.

Patch by Simon Tatham <simon.tatham@arm.com>.

llvm-svn: 305929