[RISCV] Force relocations if initial MCSubtargetInfo contains FeatureRelax (#77436)

[RISCV] Force relocations if initial MCSubtargetInfo contains FeatureRelax (#77436)

Regarding
```
.option norelax
j label
.option relax
// relaxable instructions
// For assembly input, RISCVAsmParser::ParseInstruction will set ForceRelocs (https://reviews.llvm.org/D46423).
// For direct object emission, ForceRelocs is not set after https://github.com/llvm/llvm-project/pull/73721
label:
```

The J instruction needs a relocation to ensure the target is correct
after linker relaxation. This is related a limitation in the assembler:
RISCVAsmBackend::shouldForceRelocation decides upfront whether a
relocation is needed, instead of checking more information (whether
there are relaxable fragments in between).

Despite the limitation, `j label` produces a relocation in direct object
emission mode, but was broken by #73721 due to the shouldForceRelocation
limitation.

Add a workaround to RISCVTargetELFStreamer to emulate the previous
behavior.

Link: https://github.com/ClangBuiltLinux/linux/issues/1965