The RISC-V assembler needs the target ABI because it defines a flag of the the ELF file, as described in [1].
Make clang (the driver) to pass the target ABI to -cc1as in exactly the same way it does for -cc1.
Currently -cc1as knows about -target-abi but is not handling it. Handle it and pass it to the MC layer via MCTargetOptions.
[1] https://github.com/riscv/riscv-elf-psabi-doc/blob/master/riscv-elf.md#file-header