Retry the LLVM download in install_qnn_sdk.sh on dropped connections

setup_libcpp downloads a 575 MB LLVM release from GitHub with
`curl --retry 3`. Plain --retry only retries errors curl considers
transient, such as timeouts and 5xx responses. A connection reset in the
middle of the transfer (curl error 56) is not one of them, so the first
dropped connection fails the whole job:

    curl: (56) OpenSSL SSL_read: Connection reset by peer, errno 104
    Error: Failed to download LLVM

This failed test-qnn-delegate-linux and test-llama-runner-qnn-linux on
main twice in the last day. Add --retry-all-errors, as the other CI
download scripts in this repository already do.

Test plan: shell syntax check only. The flag needs curl 7.71 or newer;
the CI images run Ubuntu 22.04 with curl 7.81.

Author: shoumikhin

backends/qualcomm/scripts/install_qnn_sdk.sh

@@ -232,7 +232,7 @@ setup_libcpp() {
   LLVM_URL="https://github.com/llvm/llvm-project/releases/download/llvmorg-${LLVM_VERSION}/${BASE_NAME}.tar.xz"
 
   echo "Downloading LLVM from ${LLVM_URL}"
-  curl --retry 3 -fLO "${LLVM_URL}" || {
+  curl --retry 3 --retry-all-errors -fLO "${LLVM_URL}" || {
       echo "Error: Failed to download LLVM"
       exit 1
   }