[ARM] Support .reloc *, R_ARM_NONE, *

R_ARM_NONE can be used to create references among sections. When
--gc-sections is used, the referenced section will be retained if the
origin section is retained.

Add a generic MCFixupKind FK_NONE as this kind of no-op relocation is
ubiquitous on ELF and COFF, and probably available on many other binary
formats. See D62014.

Reviewed By: peter.smith

Differential Revision: https://reviews.llvm.org/D61992

llvm-svn: 360980
diff --git a/llvm/lib/Target/ARM/MCTargetDesc/ARMELFObjectWriter.cpp b/llvm/lib/Target/ARM/MCTargetDesc/ARMELFObjectWriter.cpp
index a93202b..1d51a96 100644
--- a/llvm/lib/Target/ARM/MCTargetDesc/ARMELFObjectWriter.cpp
+++ b/llvm/lib/Target/ARM/MCTargetDesc/ARMELFObjectWriter.cpp
@@ -143,6 +143,8 @@
   default:
     Ctx.reportFatalError(Fixup.getLoc(), "unsupported relocation on symbol");
     return ELF::R_ARM_NONE;
+  case FK_NONE:
+    return ELF::R_ARM_NONE;
   case FK_Data_1:
     switch (Modifier) {
     default: