[RISCV] Pass -target-abi to -cc1as

The RISC-V assembler needs the target ABI because it defines a flag of 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

Differential Revision: https://reviews.llvm.org/D59298

llvm-svn: 356981