[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