scripts: template_setup_posix: Add Bash 3.x support on M1 Mac

The macOS default Bash 3.x (`/bin/bash`) sets the `HOSTTYPE` variable
to `arm64` instead of `aarch64` on the M1 Macs, causing the setup
script to try fetching the toolchain tarballs from an incorrect URL.

This commit updates the setup script to re-set the `HOSTTYPE` variable
to `aarch64` if `arm64` is set by the Bash.

Signed-off-by: Stephanos Ioannidis <root@stephanos.io>

scripts/template_setup_posix

@@ -185,8 +185,17 @@ version=$(<sdk_version)
 
 # Resolve host type
 case ${OSTYPE} in
-  linux-gnu*)  host="linux-${HOSTTYPE}" ;;
-  darwin*)     host="macos-${HOSTTYPE}" ;;
+  linux-gnu*)
+    host="linux-${HOSTTYPE}"
+    ;;
+  darwin*)
+    # Bash 3.x on AArch64 Darwin sets HOSTTYPE to 'arm64'
+    if [ "${HOSTTYPE}" = "arm64" ]; then
+      HOSTTYPE="aarch64"
+    fi
+
+    host="macos-${HOSTTYPE}"
+    ;;
   *)
     echo "ERROR: Unsupported host operating system"
     exit 5