[ELF] Handle SHT_RISCV_ATTRIBUTES similarly to SHT_ARM_ATTRIBUTES
Currently we treat SHT_RISCV_ATTRIBUTES like a normal section and concatenate all such input sections, yielding invalid output unless only a single attributes section is present in the input. Instead, pick the first as with SHT_ARM_ATTRIBUTES. We do not currently need to condition our behaviour on the contents, unlike Arm. In future, we should both do stricter validation of the input and merge all sections together to ensure we have, for example, the full arch string requirement, but this rudimentary implementation is good enough for most common cases.