[LLD][RISCV] Report error for unsatisfiable RISCV_ALIGN (llvm#74121)

If we have a RISCV_ALIGN relocation which can't be satisfied with the
available space provided, report an error rather than silently
continuing with a corrupt state.

For context, llvm#73977 fixes an
LLD bug which can cause this effect, but that's not the only source of
such cases.

Another is our hard-to-fix set of LTO problems. We can have a single
function which was compiled without C in an otherwise entirely C module.
Until we have all of the mapping symbols and related mechanisms
implemented, this case can continue to arise.

I think it's very important from a user interface perspective to have
non-assertion builds report an error in this case. If we don't report an
error here, we can crash the linker (due to the fatal error at the
bottom of the function), or if we're less lucky silently produce a
malformed binary.

There's a couple of known defects with this patch.

First, there's no test case. I don't know how to write a stable test
case for this that doesn't involve hex editing an object file, or
abusing the LTO bug that we hope to fix.

Second, this will report an error on each relax iteration. I explored
trying to report an error only once after relaxation, but ended up
deciding I didn't have the context to implement it safely.

I would be thrilled if someone more knowledgeable of this code wants to
write a better version of this patch, but in the meantime, I believe we
should land this to address the user experience problem described above.

Author: preames

lld/ELF/Arch/RISCV.cpp

@@ -731,8 +731,14 @@ static bool relax(InputSection &sec) {
       const uint64_t align = PowerOf2Ceil(r.addend + 2);
       // All bytes beyond the alignment boundary should be removed.
       remove = nextLoc - ((loc + align - 1) & -align);
-      assert(static_cast<int32_t>(remove) >= 0 &&
-             "R_RISCV_ALIGN needs expanding the content");
+      // If we can't satisfy this alignment, we've found a bad input.
+      if (LLVM_UNLIKELY(static_cast<int32_t>(remove) < 0)) {
+        errorOrWarn(getErrorLocation((const uint8_t*)loc) +
+                    "insufficient padding bytes for " + lld::toString(r.type) +
+                    ": " + Twine(r.addend) + " bytes available "
+                    "for requested alignment of " + Twine(align) + " bytes");
+        remove = 0;
+      }
       break;
     }
     case R_RISCV_CALL: