objtool: Provide elf_write_{insn,reloc}()

This provides infrastructure to rewrite instructions; this is
immediately useful for helping out with KCOV-vs-noinstr, but will
also come in handy for a bunch of variable sized jump-label patches
that are still on ice.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>