* Add support for riscv64:
- Always link with -latomic on riscv64.
- patches/clang-riscv64-multiarch.diff: add multiarch paths for riscv64.
- patches/clang-riscv64-rv64gc.diff: default to lp64d ABI and rv64gc ISA.
- patches/clang-riscv64-hf-abi.diff: backport riscv64 hard-float support
from upstream.
- patches/libcxx/libcxx-riscv64-cycletimer.diff: backport riscv64
cycletimer support from upstream.
