[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 #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: ClangBuiltLinux/linux#1965

Author: MaskRay

llvm/lib/Target/RISCV/MCTargetDesc/RISCVELFStreamer.cpp

@@ -37,6 +37,13 @@ RISCVTargetELFStreamer::RISCVTargetELFStreamer(MCStreamer &S,
   auto &MAB = static_cast<RISCVAsmBackend &>(MCA.getBackend());
   setTargetABI(RISCVABI::computeTargetABI(STI.getTargetTriple(), Features,
                                           MAB.getTargetOptions().getABIName()));
+  // `j label` in `.option norelax; j label; .option relax; ...; label:` needs a
+  // relocation to ensure the jump target is correct after linking. This is due
+  // to a limitation that shouldForceRelocation has to make the decision upfront
+  // without knowing a possibly future .option relax. When RISCVAsmParser is used,
+  // its ParseInstruction may call setForceRelocs as well.
+  if (STI.hasFeature(RISCV::FeatureRelax))
+    static_cast<RISCVAsmBackend &>(MAB).setForceRelocs();
 }
 
 RISCVELFStreamer &RISCVTargetELFStreamer::getStreamer() {

llvm/test/CodeGen/RISCV/option-relax-relocation.ll

@@ -0,0 +1,31 @@
+;; With +relax, J below needs a relocation to ensure the target is correct
+;; after linker relaxation. See https://github.com/ClangBuiltLinux/linux/issues/1965
+
+; RUN: llc -mtriple=riscv64 -mattr=-relax -filetype=obj < %s \
+; RUN:     | llvm-objdump -d -r - | FileCheck %s
+; RUN: llc -mtriple=riscv64 -mattr=+relax -filetype=obj < %s \
+; RUN:     | llvm-objdump -d -r - | FileCheck %s --check-prefixes=CHECK,RELAX
+
+; CHECK:        j       {{.*}}
+; RELAX-NEXT:           R_RISCV_JAL  {{.*}}
+; CHECK-NEXT:   auipc   ra, 0x0
+; CHECK-NEXT:           R_RISCV_CALL_PLT     f
+; RELAX-NEXT:           R_RISCV_RELAX        *ABS*
+; CHECK-NEXT:   jalr    ra
+
+define dso_local noundef signext i32 @main() local_unnamed_addr #0 {
+entry:
+  callbr void asm sideeffect ".option push\0A.option norvc\0A.option norelax\0Aj $0\0A.option pop\0A", "!i"() #2
+          to label %asm.fallthrough [label %label]
+
+asm.fallthrough:                                  ; preds = %entry
+  tail call void @f()
+  br label %label
+
+label:                                            ; preds = %asm.fallthrough, %entry
+  ret i32 0
+}
+
+declare void @f()
+
+attributes #0 = { nounwind "target-features"="-c,+relax" }