[X86] Support .reloc *, R_{386,X86_64}_NONE, *

This 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.

See R_MIPS_NONE (D13659), R_ARM_NONE (D61992), R_AARCH64_NONE (D61973) for similar changes.

Reviewed By: rnk

Differential Revision: https://reviews.llvm.org/D62014

llvm-svn: 360983