work around build errors in T-head qemu (see xuantie-qemu issue 7)

.github/workflows/c910v.yml

@@ -52,6 +52,7 @@ jobs:
           wget https://github.com/revyos/qemu/commit/5164bca5a4bcde4534dc1a9aa3a7f619719874cf.patch
           cd qemu
           patch -p1 < ../5164bca5a4bcde4534dc1a9aa3a7f619719874cf.patch
+          export CXXFLAGS="-Wno-error"; export CFLAGS="-Wno-error"
           ./configure --prefix=$GITHUB_WORKSPACE/qemu-install --target-list=riscv64-linux-user --disable-system
           make -j$(nproc)
           make install