[COFF] Emit @feat.00 on 64-bit and set the CFG bit when emitting guardcf tables

The 0x800 bit in @feat.00 needs to be set in order to make LLD pick up
the .gfid$y table. I believe this is fine to set even if we don't emit
the instrumentation.

We haven't emitted @feat.00 on 64-bit before. I see that MSVC does emit
it, but I'm not entirely sure what the default value should be. I went
with zero since that seems as safe as not emitting the symbol in the
first place.

Differential Revision: https://reviews.llvm.org/D52235

llvm-svn: 342532