r0 = 1 silently assembles to nothing on sbf-solana-solana instead of emitting an error

## Problem

For the following assembly:

```asm
.globl entrypoint

entrypoint:
r0 = 0x1
exit
```

when assembled with:

```sh
clang -target sbf-solana-solana -c ...
```

the assembler does not report an error. Instead, the resulting object contains only the `exit` instruction, while `r0 = 0x1` is interpreted as a symbol assignment and ended up appearing in the `.symtab`.

For the sbpf target, `r0 = 0x1` is not valid syntax, so this input should be rejected rather than silently interpreted as a symbol assignment.

Although this syntax is supported by the bpf/llvm backend, sbpf is a specialized target with its own assembly syntax and constraints. So in this case the assembler should therefore explicitly diagnose this case as invalid input.