add support for gcc cross-build for linux targets

[UNTESTED]

_build.sh

@@ -572,11 +572,17 @@ build_single_target() {
       # Include CRT type in Linux triplets, to make it visible in
       # the curl version banner.
       _TRIPLET="${_machine}-pc-linux-${_CRT}"
+      # Short triplet used on the Linux filesystem
+      _TRIPLETSH="${_machine}-linux-gnu"
 
       if [ "${unamem}" != "${_machine}" ] && [ "${_CC}" = 'gcc' ]; then
-        # TODO: Implement cross-builds with gcc
-        echo "! WARNING: Linux cross-buils require llvm/clang. Skipping."
-        return
+        # https://packages.debian.org/testing/arm64/gcc-x86-64-linux-gnu/filelist
+        # https://packages.debian.org/testing/arm64/binutils-x86-64-linux-gnu/filelist
+        # /usr/bin/x86_64-linux-gnu-gcc
+        # https://packages.debian.org/testing/amd64/gcc-aarch64-linux-gnu/filelist
+        # https://packages.debian.org/testing/amd64/binutils-aarch64-linux-gnu/filelist
+        # /usr/bin/aarch64-linux-gnu-gcc
+        _CCPREFIX="${_TRIPLETSH}-"
       fi
 
       _RUN_BIN='echo'

_ci-linux-debian.sh

@@ -18,9 +18,17 @@ extra=''
 if [[ "${CW_CONFIG:-}" = *'linux'* ]]; then
   extra="${extra} checksec"
   if [ "$(uname -m)" = 'aarch64' ]; then
-    extra="${extra} libgcc-13-dev-amd64-cross libstdc++-13-dev-amd64-cross"
+    if [[ "${CW_CONFIG:-}" = *'gcc'* ]]; then
+      extra="${extra} gcc${CW_CCSUFFIX}-x86-64-linux-gnu"
+    else
+      extra="${extra} libgcc-13-dev-amd64-cross libstdc++-13-dev-amd64-cross"
+    fi
   else
-    extra="${extra} libgcc-13-dev-arm64-cross libstdc++-13-dev-arm64-cross"
+    if [[ "${CW_CONFIG:-}" = *'gcc'* ]]; then
+      extra="${extra} gcc${CW_CCSUFFIX}-aarch64-linux-gnu"
+    else
+      extra="${extra} libgcc-13-dev-arm64-cross libstdc++-13-dev-arm64-cross"
+    fi
   fi
 fi