cranelift-native: Detect RISC-V extensions using /proc/cpuinfo (#6192)

* cranelift-native: Move riscv to separate module

* cranelift-native: Read /proc/cpuinfo to parse RISC-V extensions

* ci: Add QEMU cpuinfo emulation patch

This patch emulates the /proc/cpuinfo interface for RISC-V. This
allows us to do feature detection for the RISC-V backend.

It has been queued for QEMU 8.1 so we should remove it as soon as
that is available.

* ci: Enable QEMU RISC-V extensions

* cranelift-native: Cleanup ISA string parsing

Co-Authored-By: Jamey Sharp <jsharp@fastly.com>

* cranelift-native: Rework `/proc/cpuinfo` parsing

Co-Authored-By: Jamey Sharp <jsharp@fastly.com>

---------

Co-authored-by: Jamey Sharp <jsharp@fastly.com>

.github/workflows/main.yml

@@ -393,7 +393,7 @@ jobs:
     - uses: actions/cache@v3
       with:
         path: ${{ runner.tool_cache }}/qemu
-        key: qemu-${{ matrix.target }}-${{ env.QEMU_BUILD_VERSION }}
+        key: qemu-${{ matrix.target }}-${{ env.QEMU_BUILD_VERSION }}-patchcpuinfo
       if: matrix.target != '' && matrix.os == 'ubuntu-latest'
     - name: Install cross-compilation tools
       run: |
@@ -426,6 +426,7 @@ jobs:
         # quickly.
         curl https://download.qemu.org/qemu-$QEMU_BUILD_VERSION.tar.xz | tar xJf -
         cd qemu-$QEMU_BUILD_VERSION
+        patch -p1 < $GITHUB_WORKSPACE/ci/qemu-cpuinfo.patch
         ./configure --target-list=${{ matrix.qemu_target }} --prefix=${{ runner.tool_cache}}/qemu --disable-tools --disable-slirp --disable-fdt --disable-capstone --disable-docs
         ninja -C build install
         touch ${{ runner.tool_cache }}/qemu/built