RISC-V Fixes for 4.17-rc3

This pull request contains three small fixes related to the RISC-V port
that I'd like to target for 4.17-rc3:

* A Kconfig cleanup to select DMA_DIRECT_OPS instead of redefining it in
  arch/riscv.
* The removal of asm/handle_irq.h, which doesn't exist, from our arch
  header list.
* The addition of "-no-pie" the link rules for our VDSO-related files,
  which fixes the build on systems where PIE is enabled by default.
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