Merge remote-tracking branch 'remotes/alistair/tags/pull-riscv-to-apply-20200702...

This PR contains two patches to improve PLIC support in QEMU.

It also contains one patch that fixes CLINT accesses for RISC-V. This
fixes a regression for most RISC-V boards.

The rest of the PR is adding support for the v0.7.1 RISC-V vector
extensions. This is experimental support as the vector extensions are
still in a draft state.

This is a v2 pull request that has fixed the building on big endian
machines failure.

# gpg: Signature made Thu 02 Jul 2020 17:21:54 BST
# gpg:                using RSA key F6C4AC46D4934868D3B8CE8F21E10D29DF977054
# gpg: Good signature from "Alistair Francis <alistair@alistair23.me>" [full]
# Primary key fingerprint: F6C4 AC46 D493 4868 D3B8  CE8F 21E1 0D29 DF97 7054

* remotes/alistair/tags/pull-riscv-to-apply-20200702-1: (64 commits)
  target/riscv: configure and turn on vector extension from command line
  target/riscv: vector compress instruction
  target/riscv: vector register gather instruction
  target/riscv: vector slide instructions
  target/riscv: floating-point scalar move instructions
  target/riscv: integer scalar move instruction
  target/riscv: integer extract instruction
  target/riscv: vector element index instruction
  target/riscv: vector iota instruction
  target/riscv: set-X-first mask bit
  target/riscv: vmfirst find-first-set mask bit
  target/riscv: vector mask population count vmpopc
  target/riscv: vector mask-register logical instructions
  target/riscv: vector widening floating-point reduction instructions
  target/riscv: vector single-width floating-point reduction instructions
  target/riscv: vector wideing integer reduction instructions
  target/riscv: vector single-width integer reduction instructions
  target/riscv: narrowing floating-point/integer type-convert instructions
  target/riscv: widening floating-point/integer type-convert instructions
  target/riscv: vector floating-point/integer type-convert instructions
  ...

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>