Patch qemu in CI to fix madvise semantics. (#3770)

We currently skip some tests when running our qemu-based tests for
aarch64 and s390x. Qemu has broken madvise(MADV_DONTNEED) semantics --
specifically, it just ignores madvise() [1].

We could continue to whack-a-mole the tests whenever we create new
functionality that relies on madvise() semantics, but ideally we'd just
have emulation that properly emulates!

The earlier discussions on the qemu mailing list [2] had a proposed
patch for this, but (i) this patch doesn't seem to apply cleanly anymore
(it's 3.5 years old) and (ii) it's pretty complex due to the need to
handle qemu's ability to emulate differing page sizes on host and guest.

It turns out that we only really need this for CI when host and guest
have the same page size (4KiB), so we *could* just pass the madvise()s
through. I wouldn't expect such a patch to ever land upstream in qemu,
but it satisfies our needs I think. So this PR modifies our CI setup to
patch qemu before building it locally with a little one-off patch.

[1]
https://github.com/bytecodealliance/wasmtime/pull/2518#issuecomment-747280133

[2]
https://lists.gnu.org/archive/html/qemu-devel/2018-08/msg05416.html

.github/workflows/main.yml

@@ -253,7 +253,7 @@ jobs:
     - uses: actions/cache@v2
       with:
         path: ${{ runner.tool_cache }}/qemu
-        key: qemu-${{ matrix.target }}-${{ env.QEMU_BUILD_VERSION }}
+        key: qemu-${{ matrix.target }}-${{ env.QEMU_BUILD_VERSION }}-patchmadvise2
       if: matrix.target != '' && matrix.os == 'ubuntu-latest'
     - name: Install cross-compilation tools
       run: |
@@ -286,6 +286,7 @@ jobs:
         # quickly.
         curl https://download.qemu.org/qemu-$QEMU_BUILD_VERSION.tar.xz | tar xJf -
         cd qemu-$QEMU_BUILD_VERSION
+        patch -p1 < $GITHUB_WORKSPACE/ci/qemu-madvise.patch
         ./configure --target-list=${{ 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