[LLD][COFF] Ignore /kernel flag

There exists some description of the flag from Microsoft, but not sure if
there's more to it. We ignore the flag for now until we find out more about it.

https://docs.microsoft.com/en-us/cpp/build/reference/kernel-create-kernel-mode-binary?view=msvc-170

Reviewed By: thieta, hans

Differential Revision: https://reviews.llvm.org/D128238

lld/COFF/Options.td

@@ -296,6 +296,7 @@ def nologo : F<"nologo">;
 def throwingnew : F<"throwingnew">;
 def editandcontinue : F<"editandcontinue">;
 def fastfail : F<"fastfail">;
+def kernel : F<"kernel">;
 def pdbcompress : F<"pdbcompress">;
 
 def delay : P_priv<"delay">;