[MC][ELF] Emit a relocation if target is defined in the same section and is non-local

For a target symbol defined in the same section, currently we don't emit
a relocation if VariantKind is VK_None (with few exceptions like RISC-V
relaxation), while GNU as emits one. This causes program behavior
differences with and without -ffunction-sections, and can break intended
symbol interposition in a -shared link.

```
.globl foo
foo:
  call foo      # no relocation. On other targets, may be written as b foo, etc
  call bar      # a relocation if bar is in another section (e.g. -ffunction-sections)
  call foo@plt  # a relocation
```

Unify these cases by always emitting a relocation. If we ever want to
optimize `call foo` in -shared links, we should emit a STB_LOCAL alias
and call via the alias.

ARM/thumb2-beq-fixup.s: we now emit a relocation to global_thumb_fn as GNU as does.
X86/Inputs/align-branch-64-2.s: we now emit R_X86_64_PLT32 to foo as GNU does.

ELF/relax.s: rewrite the test as target-in-same-section.s .
We omitted relocations to `global` and now emit R_X86_64_PLT32.
Note, GNU as does not emit a relocation for `jmp global` (maybe its own
bug). Our new behavior is compatible except `jmp global`.

Reviewed By: peter.smith

Differential Revision: https://reviews.llvm.org/D72197

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