[RISCV][MC] Adjust conditions to emit R_RISCV_ADD*/R_RISCV_SUB* pairs

D132262 tried to simplify `IsMetadataOrEHFrameSection` originally introduced in
D127549 but caused a regression as `.quad` directives in

```
.section .note,"a",@note; note:
.quad extern-note    # extern is undefined

.section .rodata,"a",@progbits; rodata:
.quad extern-rodata  # extern is undefined

.section .nonalloc,"",@progbits; nw:
.quad extern-nw
```

are incorrectly rejected: these differences may be link-time constants and
are allowed in GNU assembler and LLVM MC's non-RISC-V ports.

Relax the conditions to allow these cases. For A-B, A may be defined later, but
this requiresFixups call has to eagerly make a decision. For now, emit ADD/SUB
unless A is `.L*`. This euristic handles many temporary label differences for
.debug_* and .apple_types sections. Ideally we should delay the decision of
PC-relative vs ADD/SUB until A is defined.

Reviewed By: compnerd

Differential Revision: https://reviews.llvm.org/D145474